Asklemmy

Nonce. I've always used it just like Dunce. Turns out it does in fact not mean the same and instead means pedo.

Turns out pseudo is not spelled like sudo.

Proscribe and prescribe.

Penultimate. I used it as though it referred to the last thing rather than the second to last thing.

Inflammable means flammable? What a country

I don't use it wrong because I don't use it but to me "mirth" feels like it has a negative connotation even though I know it means joyful.

In German we use the English term 'understatement' in some occasions, e.g. 'his dressing style is a clear understatement...'. My brain somehow tied the meaning to something like 'understanding', maybe due to the similarities of both words. For decades it was clear to me that someone dressing like that were dressed to the point and 'making a clear statement'. Now that I've checked the real meaning, I'm completely puzzled when and how to use the term and what I've misinterpreted all the years...

"Sorry."

Terrence Thatcher, T. May, Terroris, Ttruss;
Terry-Tories, Terry-Tories, make a fuss!

It also gives a different meaning to the citizens of British Overseas Tories!

Since OP is in the UK, I can pull out “nonplussed.” Current American usage of the word is a lack of surprise or general acceptance. I am nonplussed when news arrives that another politician was caught in a sex scandal. Non-American usage is complete surprise and an inability to act. The Scot was nonplussed when the drunk American vomited noisily on his shoes.

Edit: I am firmly in the “general acceptance” camp and usually have to process for a second or two when someone uses it in its traditional sense.

When I was younger I thought concur meant disagree.

It took me until graduate school to learn that "mortified" is not another word for "scared"/"fearful"

It still looks that way to me what with mort in there!

It also took me a long time to realize that the word "awry", which I read often in books and inferred its meaning, and "ah-rai" were the same thing. I thought awry was pronounced "aw-ree" and it was just a synonym for "ah-rai".

The word "nauseous" is parallel to "noxious" and means "causing nausea". If you're experiencing nausea, you're nauseated -- the thing that made you nauseated is nauseous.

Inconceivable!

Not a word but I thought the idiom toe the line meant basically the same as push the envelope. As in you're testing the boundaries of something by walking right up to the line and nudging it with your toe to move it further.

Turns out it means pretty much the opposite, essentially the same as fall in line.

Callow. It just means immature but I somehow got it in my head that it meant cowardly.

Man, I'm learning some weird stuff about British people. I've never heard of encephalitis being associated with HIV or a nonce being any kind of person.

ages ago, i spent something like half a year thinking there was a word "appericate". it was an odd one, since i only ever saw it in print, and from context it was clear that it meant the same thing as "appreciate", which, oddly enough, i only ever heard in speech.

and then one day i stopped at an "appericate" in a book and re-read it 9 times, very slowly.

With English as my second language, the difference between terrible and terrific has always confused me.

Depreciate.

I'm going to Depreciate this computer. That means to get its loss of value on taxes....not to shut it down. The word I meant was Deprecate