When I lived in Ireland for a few months, people would ask me straight to my face if I was “on the phone.” I would think, “what, are you BLIND? Does it look like I’m on the phone? I’m standing out here on the street, talking to you!” (This was before cell phones).
Turns out “on the phone” meant “in possession of a land line at one’s residence” – i.e., “on” the phone system. This was just before the economy took off in the 1990s, so many people didn’t yet have home telephone service.
It’s a slang term for “Fuck(ing someone)” in New Zealand too. It can also mean something’s broken - “The TV’s rooted” means the appliance in question is not functioning (and may be well beyond repair, such is the nature of its non-function).
It’s also been my experience that a lot of that “Classic Aussie Slang” doesn’t actually get used that much by people in cities but you do hear it from time to time in the bush (or people that have recently moved from the bush). Having said that, I’ve lived here so long I’m probably used to it so it doesn’t even register unless it sounds like something from an old Dad and Dave serial. 
A foreign language one - the Malay word “Air” means “Water”, not air. So yes, there is liquid in the bottle labelled “Air”. (There’s probably a joke about turning air into water somewhere, too!)
“Root” does indeed mean to have sex, or to break or be broken, and so “root” is completely interchangeable with “fuck” in all uses I can think of. “It’s fucked/it’s rooted” basically means it’s broken beyond repair, with the latter being marginally less offensive than the former.
It goes without saying that it made me uncomfortable when Div said he rooted my phone for me, and I resort to the American pronunciation of router so I don’t have to call the thing a “rooter” all day long.
One between Spanish and Italian which is famous enough that nowadays it trips relatively few people: burro is donkey in Spanish, butter or lard in Italian.
Similar (although not especially related is the Scottish use of ‘stay’ - In England, we’ll ask “where do you live?”, in Scotland, it’s (not sure how universally) “where do you stay?” - and English person hearing the Scots usage is liable to misinterpret stay as meaning temporary residence (such as a stay in a hotel).
Not sure if it’s universal, but it’s very widespread, in my experience. In some parts, the word “bide” is used similarly.
From upthread, “loon” meaning “boy” is a north-east Scotland usage, basically Aberdeen and around that area. It’s not used outside that area, although most will know what it means. The equivalent for “girl” is “quine”.
In the USA the phrase “I could care less” means “I do not care”, but in the UK it would imply that you do care (but perhaps not very much, “I *could *care less, but it would be a struggle”).
It’s not really a phrase we use in the UK (we’d say “I couldn’t care less” to indicate a lack of concern), but it confused me a few times when I heard Americans say it.
Is there an implied missing question mark? i.e. “Do you really think I could care less about this??”
Otherwise it seems like it means the precise opposite to the intended meaning.
Nope, it just degenerated to its current form through popular usage. Now it’s illogical if you think hard about it, but people rarely do – they just say it, and the intended meaning is understood. Language can be funny that way.
It’s supposed to be sarcastic, although it’s become so common now that I don’t think people even think much about what it literally means. About 25 years ago it was the sort of thing obnoxious teenagers said in a flat voice to adults who bored them.
In the US, a “vest” is a waistcoat; in Britain and Ireland it is an undergarment (like a t-shirt worn under your shirt).
The word “pavement” in Britain and Ireland refers to the sidewalk where pedestrians walk, whereas in the US (I believe) it means the road surface itself, where the cars drive.
“Produce” in Britain and Ireland means anything produced, such as manufactured goods, whereas in the US it seems to refer to fruit and vegetables.
“Middle class” is used subtly differently on the two sides of the Atlantic; in Britain and Ireland it refers to professional classes and has connotations of privilege and affluence, while in the US it refers to a wider social group and has connotations of “ordinary decent people”.
Believe it or not, I was genuinely caught out and misled by that, on an early visit to Japan (probably about 15 years ago). I ordered “taco rice” thinking I was going to get rice with octopus. I was most of the way through it wondering why I hadn’t found any octopus yet - maybe they were minced up really small? - before my wife explained to me that it was taco rice, not octopus rice.
You would think the taco shell would have been a clue, but I wasn’t very familiar with the concept of tacos at that time.
In my neck of the USA, at least, the part where cars drive is almost always referred to as “the road” Someone saying pavement would get a lot of raised eyebrows. A bike trail or walking path that’s not dirt may be called “paved” and a road getting fixed may be said to be “repaved” but the word pavement itself is not really in great use in the US, again, at least, in my area.
We’re gonna sink some piss at the hotel then maybe root some Sheilas, fair dinkum.
Pants also means trousers in large parts of Britain.
I well remember the hilarity of Happy Days and the dog named “Spunky” and they kept saying it with each time it grew unbearably funny, to us kids, until we nearly exploded. To us “Spunk” mean sperm. And it also had a character called Mr Wanker, the conjunction of the two was unbearably funny, wank means to the UK to masturbate.
An American phrase i find odd its “to luck out” as i assumed it meant to have no luck but apparently it means the very opposite.
“That kid is full of spunk!”, paraphrasing.
That’s a matter for police intervention here.
I have most probably posted this before but it needs repeating: What a load of codswallop, pet! Words that can be confusing and embarrassing in the UK & US.
Talking about somthing other that English, there are false friends between Swedish and Danish/Norwegian and between Swedish in Sweden and Swedish in Finland. Some examples:
rolig: Means fun or funny in Swedish and calm in Danish/Norwegian. There are many stories of Swedes in the other countries who have asked taxi drivers to be taken to a rolig place and ended up in a cemetary.
laga: The original meaning is make, which is still used in Finland, but in Sweden it has changed to mend/repair except for laga mat = cook.
In the UK a tank top is a (usually woollen) buttonless sleeveless top worn over another top, usually a collared shirt. In the US it’s a buttonless sleeveless top worn either as an undershirt or as a top on its own.
That latter description is what we in the UK would call a vest. In the US, a vest is a (usually leather, silk or suede) button-up item worn over a shirt. That, in the UK, is a waistcoat, which I believe is also used in the US for the same item.
Braces, in the UK, can be either a dental brace (but you’d nearly always hear it as braces, not a brace) or elastic/fabric items hooked onto trousers to hold them up in lieu of a belt. In the US, that latter description fits what you call suspenders. However, in the UK, suspenders are fabric strips attached to a suspender belt around a thigh and used to hold up stockings.
I was genuinely confused a few times as a kid to read about a man wearing suspenders. Especially if he was getting all dressed up and for this wore suspenders and a vest.
Fancy dress also has a different meaning - here it can be “a dress that is fancy,” depending on context and intonation, but as a phrase it usually means “costume,” as in the type you’d wear to a costume party (a fancy dress party). I don’t know how common the first meaning (“posh clothing”) is in the US, but I certainly had a friend from Florida tell me she was going to a party that was fancy dress and she then got very confused about my suggestions for an outfit.
It has both meanings in British English. The “good amount” meaning is more widespread, I think, but both are in use.
If you say “quite smart,” with the stress slightly on the smart, it means the same as in the US - “rather smart.” However, quite smart, with the stress (more strongly) on the quite, means not actually all that smart really.
Just saw another one used elsewhere, and was reminded: anymore is another one that has caused confusion in real life for me - in England we only use it in the negative, so a sentence like “he’s living in Boston anymore” is either completely meaningless (because the “nowadays” meaning simply isn’t used here and it’s not obvious from context) or interpreted as meaning he’s not living there, and the word “not” got left out or something.
So you can give someone a British compliment, that is, a witty putdown, and it’d be difficult for them to detect it and call you on it.
Are there any other such tricks up Limey sleeves?