The best thing to do when you are dumped by your girlfriend. go out and pull an old bird. the game consists of a pack of young males who attempt to shag the brains out of an attractive OLDER woman. Fuelled by booze these young males are letting themselves in for months of ribbing. Another problem is; when on boxing day, this one night stand turns up and is your mum's best friend, or dad's ex.
Alternatively, grab-a-granny could mean mugging a female pensioner.
Mark: "Oh no, i'm crushed. My janine has dumped me. Oh, woe is me."
Keith: "No worries bud. We're out tonight and we'll partake in grab-a-granny"
Baz: "I need some cash for some crack and a whore."
Bez: "Ok, let's grab-a-granny, and claim it was noel edmonds."