An exclamation used in order to stop someone from turning a statement into a sexual innuendo; mainly used to prevent the use of the 'That's what she said' phrase.
Has since developed into a sort of game by British university students, in which whoever happens to say an accidental innuendo has to protect themself with 'Bantage!' before a friend can say 'That's what she said!'
Jadeness: 'Man, I'm so tired. The guys in my flat kept me up all night...'
Lexie: 'That's what-'
Lexie: 'Dammit! Beat me.'
Bynski: 'Haha, gotta be faster next time Lex.'