Decimate has morphed from “reduce by one-tenth” to “wipe off the face of the earth.” So instead of having 90% of what you started with, you have nothing.
For some reason, this usage irritates me to no end. I suppose I should just get over it.
With.
I believe it used to mean “against” or “versus*”. It is still used in this sense in “fighting with” etc, but has generally come to mean the opposite.
*In Australia, a lot of kids these days have mistaken “versus” for “verses”, and a sportsman can be asked “Who are you versing this weekend?” Is that the same elsewhere? “Verse” seems to be gaining traction here.
Liberal is one that gets me lately. It’s gained a connotation that has nothing to do with the previously popular meaning of “open to new ideas”.
“Magazine” derives from “makhzan”, an arabic word meaning “a storehouse for grain” 9as I recall from mt World Book Encyclopedia). I don’t know anywhere that use persists in English, but the military uses of "magazine: as a place to store gunpowder, or as the place where a firearm keeps its unexpended rounds obviously derives from that. It’s more of a stretch to “magazine” as a storehouse of articles in printed form.
“Diaper” originally referred to a diagonally patterned cloth (quilted?), with the word “diaper” actually coming from the adjective for “diagonal”. The name got transferred to that sort of cloth, then to other types of cloth, and finaly to cloths used to cloth babies, probably from a desire to dignify that use. Then to the paper. plastic, and coagulant constructions that we dispose of.
“Toilet” is probably the most familiar – it used to refer to one’s cleansing ritual, and as an adjective referred to a table and water and other things, before it was applied euphemistically to our flushing chamber pots. I can easily imagine why the euphemism got applied – it’s still a “cleansing ritual” of a kind. But as a kid I couldnm’t understand why anyone would want to apply “toilet water” to themselves.
Indeed the same process is underway with ‘bathroom’.
quick, as in the quick and the dead. It doesn’t mean that the slowest get killed - quick used to mean “living”
Si
Prestige, according to OED, used to mean a sleight-of-hand trick (hence the name of the recent Hugh Jackman movie.)
Fond used to mean foolish, rather than affectionate.
Cad, Blackguard and possibly churl all derive from upper crust nicknames for the lower class.
Cunning used to mean cute
The word dollar is derived from the Dutch currency Taler. Taler (or thaler) in Dutch means valley.
Bit and picyune both come from the names of small amounts of money.
Being glamourous or charming originally meant you were under a magic spell.
In Scottish canny meant you were smart, but Uncanny meant something unknown or unknowable.
“Snob” originated in boys’ schools in England. It was a boy-invented slang word used to describe those boys who weren’t the “nobs”, or the sons of nobility. You were either a nob or a snob.
Thackeray came along and used the word to describe people who were wannabe nobility, or people who followed and fawned upon the upper classes. Then he also started including in this description the upper classes who were fixated on their own superior rank, as well.
So snob first meant lower class people, and now means anyone who thinks they are better than other folk.
Yes, “nice” used to mean that, too. According to the OED, the meanings of the word are:
There is also Sections II & III, “Phrases” and “Special Uses”, respectively, that I won’t list here. “Nice” may also be used as a verb and a noun.
Nitpick: It’s “Baranduin.”
And everybody’s already mentioned my favorite meaning-shifted words, so I’ll bestow one of my favorite bits of American Civil War slang on you: “Absquatulate,” meaning to get drunk.
You probably know this, but camera is still used for “big room” or “suite” in Italian. Stanza is used for “small room.”
My favorite recent discovery is the origin of “lieutenant”. It’s kind of obvious when you break it down into “lieu” and “tenant”. It’s someone who occupies a position in lieu of its rightful holder – i.e. a second-in-command who takes over for a leader when he’s away.
And “in camera” review is when a judge reviews documents in privacy or away from a jury. One of our judges persistently referred to it as “in camero” review, leading some to wonder if she was going to buy a Chevy sportscar for that purpose.
And the reason is, it was once believed that madness fluctuated with the phases of the moon.
I have noted a word on the way to a change in meaning. Yesterday my husband and I were talking about one of the newer SUVs that seem to be a combination of SUV and minivan. We were referring to them as “hybrids,” as in, a hybrid design of the two. It was then I realized that “hybrid,” at least in terms of cars, is coming to solely mean a gas/electric hybrid.
Ever wonder why major outranks a lieutenant, but a lieutenant general outranks a major general? The Master Speaks.
“Marlin” as in the fish comes from “marlin spike,” a tool used to work marlins (i.e., “moor lines”). The fish was obviously named for the tool, not the rope that the tool is used for.
And I think “dolphin,” which originally referred only to a mammal, became ascribed also to a type of fish due to sailor corruption of the existing word “dorado” (cf. “turtle” earlier in the thread).
I don’t have any I can think of right now, I just wanted to mention that I never did realize this until I stared at Lord of the Rings maps for far, far too long. When I saw the Baranduin, I rolled the word around in my head, rolled it around… and suddenly a light came on.
Brandywine!
It’s a teensy name that says so much about the history of the place, the time that’s passed…
…mm, I still seem to have some geek on my shoes from the Harry Potter showing. 
He did when he offered “man of the city” as the root for “policeman” in one of his Nightwatch books.