My wife, a native Spanish speaker, today asked me about why a youtuber would call themselves 'craftypants'.
I explained that -pants was added to something as synecdoche, so for example an intelligent person might be called a 'smarty-pants', a poor-humored person a 'grumpy-pants', etc.
However, I then realized that while this is common in the US, in the UK 'pants' by itself usually means what this Yank would call 'underwear', while what I call 'pants' would appear over there as 'trousers'.
How do such expressions as 'fancy-pants' sound to speakers in the UK? Are they understandable in meaning but recognizable as being North American? Does putting a -pants ending on a word remove the association with intimate clothing? Does it just sound bizarre? Do parallel expressions like grumpy-trousers exist? Are some combinations acceptable in polite speech, but others (e.g., pissy pants) not?
Edit: There is much discussion in comments, and some in answers, about whether -pants is synecdoche. That's fair, and I am happy to have the point challenged. However, that's not my question. Even if I am wrong, my question is how "-pants" sounds to UK speakers, and whether it is clear or not that it is different from "pants" (underwear).