British slang for "Holy fuck", or "Aw damn", and other similar phrases. Saying the term with a British accent makes it sound more legit, even if you aren't British.
The phrase "I'll twat you one" is closely linked to this word.
Past Tense:
Guy 1: My Grandmother was looking at my porn collection yesterday...
Guy 2: Jesus man, what did you do?!
Guy 1: I twatted her one and it was lights out.