Wanker is a pretty non-specific insult in British English, usually but not quite exclusively applied to males. It generally suggests rubbishness bordering on the pathetic, being in this sense both a literal and figurative synonym for tosser, both words originally meaning 'one who masturbates'. However, wanker is much ruder, and is sometimes used to mean the same thing as bastard.
Wanker is also a game played with those plastic things that hold four-packs of beer together. You know that if you don't snap the loops and that those things kill birds, right? Well, this solves that problem in such a way as to be highly entertaining to drunk people - speaking of which, 'wankered' means 'very drunk'. However, it is possible to turn almost noun into a word meaning 'very drunk' by sticking '-ed' on the end, so perhaps this is unremarkable.
It's a simple game: Four people - blokes, traditionally, but that's really not important - each put a finger through a loop of the plastic thing. On a pre-arranged signal - 3... 2... 1... pull - that sort of thing - all players pull. The one whose loop doesn't snap - the one who is left with the plastic dangling on their finger - is declared wanker. The wanker should then make sure the remaining little loops of the plastic thing are snapped, to destroy the last of its wildlife-strangling capability. When you've all finished another four-pack, you're ready to play again...
Finally, Wanker is a surname: Thomas Wanker writes the score for Buffy The Vampire Slayer. Also, the character Spike occasionally gets away with calling people wankers, at least in the US edition of the show - presumably either because the American censors don't realise it's a proper swear word, or because they figure it would be silly to bother censoring something that's in the credits anyway...