Just recently I was in a conversation with a number of UK mainlanders and we had a debate over what “tories” meant, apparently disproportionately ordinarily it refers to a political party and it’s not usual to use it as short for “territories” as I’ve used it (according to how the debate ended, it was half and half between them). And once again I’m reminded of how people feel to look back at their usage of a word/phrase over the years and cringe.
More tragically, me and a friend were embarrassed once upon realizing everyone was confusing “encephalitis” with “hydrocephalus” when talking to someone about their kid with hydrocephalus. Awkward because encephalitis is caused by HIV.
Likewise, only ever known it as the total surprise meaning.
I wonder if the American meaning has any relation to the seemingly common use of the phrase “I could care less”, which is similarly opposite to its (to me) correct meaning.
Oh and don’t get me started on uninterested/disinterested…
It’s also generational. My dad was very confused (dare I say nonplussed?) when I used the informal meaning. IIRC he corrected me which is what led me to realize the difference. It seems to go back to at least 2013.