Option © for “no idea what it means”.
It’s bad, I’m 41, female, from the South, it’s uncommon here, and I suspect the etymology has to do with “pants” meaning underwear in other parts of the world.
It’s bad!
Mid-thirties
English, living in Italy
Fairly common amongst my English friends.
From From A Dictionary Of Slang
“pants (!) Noun/Adj. Nonsense, rubbish, bad. From the standard British English of pants, meaning underwear; also a variation on ‘knickers’. E.g.“The first half was pants but I stayed until the end and it was actually a great film.” [1990s]
Exclam. An exclamation of annoyance or frustration. From the noun, (above).”
Bad, bad, bad.
38, Seattle.
Not common here. I used to work out of my previous employer’s London office. Learnt it there. Specifically, I asked a another male co-worker if he liked my pants. Hilarity ensued.
If “pants” means underpants in British English, is “trousers” used instead?
Not sure what you mean. The slang term comes from the British sense of pants, meaning underwear. So it wouldn’t make any sense to substitute “trousers” because that’s not what it means.
The only slang use of the word “trousers” that I’ve come across is the verb “to trouser” meaning, effectively, “to pocket”, often in an illicit or otherwise undeserved way, e.g. “The CEO trousered £3 million while the company went bankrupt.”
Oh and “bit of trouser” can be used as a male version of “bit of skirt”, i.e. hot member of the preferred sex, but it’s pretty rare. Here’s an example from a British newspaper, referring to Doctor Who:
Yes. There are (as always) regional words too, “breeks” for trousers in parts of Scotland. for example.
To the OP:
38, Scotland
It means bad. I think it was largely 1990’s teen-speak though.
But not as an alternative to the slang adjective “pants”, which I think is what Walloon meant.
pants doesn’t mean bad. it means useless (or nearly so). so a toothbrush that is being used to clean a television screen is pants. flip flops with a broken strap are pants.
I assumed he was asking if “trousers” was used to denote the outer garment, but on reflection I’m not so sure. 
Divided by a common language indeed…
I disagree - “I bought the new Radiohead album, but it’s pants” doesn’t mean that it was unplayable.
I’m not talking about slang. What is the non-slang British word for that clothing you wear on your legs.
Trousers. Possibly pantaloons.
I’ve skipped down to the Quick Reply box:
No clue what it means.
Age is mid-30s.
Location is metropolitan New Orleans.
I guess I’d have to say “uncommon” – I’ve never heard it uttered.
It means bad.
I’m 52, live in Washington state. The expression is not used in this area. The only reason I know what it means is because I spend time in the Outpost Gallifrey forum, which has a great many UK members.
Native Arizonan, 47. I’ve never heard “pants” used as slang.
20, New Mexico. I’ve heard it once or twice, and it means that the speaker is an idiot. What they’re trying to say varies; I’ve heard it mean both good or bad.
My friend always called them trousers.
I’ve probably told this story before. We had this huge mock-argument one time (at a bar, so we were drunk) because I insisted that panties, not pants, was the correct term. She insisted that panties sounded like something little girls wear, and that it sounded perverted. “I don’t wear PANTIES,” she said scathingly. I teased her by saying, “Dude, you’re wearing panties right this minute!” She looked indignant. “I’M NOT WEARING PANTIES!” she retorted back. Then both of us looked up to meet the stares of every male within hearing distance, who were all suddenly very interested in what our conversation was all about.