Lance Armstrong can't help being a twat.
My friend bought an Iphone - now he's a twat.
No-one cares about anything I say - I'm a sad twat.
Stop being a twat it's your shot.
The outward display of affection between two intimately friendly parties by extreme piss taking, savage in nature.
Usually characterised, and generally understood by the rule of thumb that the more offensive and deftly aimed that the ribbing is, the greater the respect or underlying fondness that exists.
"Sweet Jesus, Gwyneth! Your vagina is like a wizards sleeve!"
"Haha! Your twat-banter is on top form tonight Rupert."