[fluoridation]

I think your friends were just being jerks. The primary meaning of the word is indeed "jacket". It'd be like saying to someone "I need a blunt tool" and them making fun of you because you just said you need a tool to make cannabis cigarettes.

https://dle.rae.es/chaqueta

[williamdclt]

It's not because it's technically right that it's not funny. Your english example isn't particularly funny so you wouldn't have the same reaction, but you could easily imagine plenty of sentences using "cock" or "pussy" that would be.

I know that some cultures consider it rude to laugh at this sort of things when it's not the speaker's native language (brits don't even correct me when I misuse/misspeak english because they think it's rude), but for many (most?) cultures it's really not, that's not being a jerk

[citizenpaul]

I've come to the general conclusion from experiance that people whom only know their native language are at best moderately hostile towards other languages and multi-lingual people.