"twonk" - a derogatory epithet that I have heard once or twice in UK

Interesting; I'd thought twonk a fairly local, little-used epithet, RC. Definitely a favourite of mine, often self-attributed in moments of frustration!
Used pretty much as "git" (the UK noun rather than the US verb), with roughly the same degree of offensiveness. Maybe there's a dash of "twit" in there too.

Seems to me that, like wonk, twonk can be applied to people with pretensions who try to make a big impression and fail abysmally - probably because they are trying too hard.

Derogatory terms, of course, are notorious for escaping their original definitions!

I think the value of favourite derogatory terms is in their sound rather than meaning as such. The flip side of "phonaesthetics" perhaps?

(anyone interested see the "antonym for onomatopoeia" thread for Bingley's reference to phonaesthetics)