Words that don't mean the same as they used to

(Disclaimer: Language is subjective. YMMV.)

nostalgia

From Greek roots meaning roughly a longing to return home, nostalgia used to mean homesickness, but now means a longing for the past.

gay

It used to just mean "happy". Now, of course, it denotes a minority sexual orientation.

democracy

From Greek meaning "rule by the people", a system where political power is directly in the hands of the people. Nowadays it's also synonymous with the republic, where power is held by elected representatives. (I don't like this change. America is not a democracy.)

atheist

From Greek meaning "godless", a person without belief in a god. Now almost exclusively (IME) used for people who have rejected the idea of the existence of a god, as opposed to people who have merely never been introduced to the concept.

down

Apparently this all-purpose direction word used to be a longer word meaning something like "from the hill"; i.e., downhill. Now I know why "downs" such as Watership Down are actually "up".

Christian

Originally an insulting term used for those who "followed the Way" (of Jesus Christ). Now used with quite a bit of overweening pride by people who never learned not to take the Lord's name in vain.

typewriter

It used to also mean the person who ran the typewriter as well as the machine itself. Then we got the word typist. (Then we got rid of all the typewriters.) Apparently the same thing happens with other appliances, like the dishwasher.

basilisk

It was an animal first described as a snake with a kind of crown marking on its head, hence the name 'basilisk' -- 'little king' in Greek. The creature has become much more elaborate over the ages, with all kinds of quirks, like having a cock's head, and being hatched by a toad.

Semantic drift rocks.
(But it makes making an a posteriori conlang really tough.)

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ShadowNode: atheist is entirely Greek, without a drop of Latin blood in it. (Well, -ist might be Latin, but the Latins borrowed it from Greek anyway.)

Decimate : Originally meant to destroy exactly one tenth of something. By extension, this has come to mean to destroy a significantly large portion of something, but to leave the majority intact.

Recently this word has been misused to mean to destroy most of something or to destroy something entirely
eg "The Tornado decimated this quiet suburban street.."
This is wrong wrong wrong however I am sure that, semantic drift being what it is, this will become an accepted meaning before long.

The word "computer" once meant a person employed to spend their day performing mathematical operations for various purposes, usually by means of pen-and-paper.

Human (ie. pre-Computer) computers were widely used in land wars in the era between the advent of radio communications and the invention of the first digital computers. A field officer would radio his position and the estimated position of his artillery target to a field office, which would relay it to an ordnance center. A huge staff of computers would receive reports like this, dance a mathematical jig, and reply by radio with firing coordinates. Of course, by the time the necessary information reached the field officer, it was quite likely his quarry would have moved, requiring another pass through this system. This is just one example of how the necessities of modern warfare have advanced the computing state of the art.

Slut

At one point in time, several hundred years ago, the word slut meant "servant girl," and not quite exactly what it means today: a "loose" person (usually with reference to a woman).

Proof: The proof of something used to mean its test. Today this word has come to mean a valid argument of something's correctness. The old meaning of the word led to the phrase "The proof of the pudding is in the eating," which originally meant that the test of the pudding is in the eating, but taken literally today makes no sense.