Cheating on your girlfriend. Usually used as a secret code to other male friends when you pull a bird at the club and you are gonna get some.
Guy 1: Oi. is that Billy pulling that fit bird?!
Guy 2: Yeah, he's tashing on Shiela again...
Guy 1: Oh, seen
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Guy 1: Tim's given us the signal, he's tashing
Guys: OI OI! 'Aight Tim!