"""Twat"" comes from ""twatchel"" which is a literal word for tunnel - hence its use to describe the vagina specifically, and female sexual organs in general. In terms of it being  vulgar or benign, this comes down to context, class and the regional regard for female genitals. ""Fucking twat"" shouted at a soccer match might be quite offensive and lead to fisticuffs, though ""you're a twat sometimes"" might be an almost endearing jibe to a friend on another occaision.   

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