1. to have sex (with). The association with striking (as in the origin of the word 'fuck' itself) is said to suggest the masculine role in sex, but in practise the unaffectionate term can also apply to women, especially in Australian usage where it is more common than in America. In Britain 'bang' in this sense has only been widely understood since the late 1960s. It was introduced via the phrase gang bang and the following expression.

'You're banging a major witness in a case you're trying?!' (The Last Innocent Man, US film, 1987)

2. American to be an active gang member. From the parlance of Los Angeles street gangs of the late 1980s, derived from the specific sense of gangbanger.

He's been banging for two years now.

3. to do something stupid. An item of street slang in London since 2000.