Used to be funny generally and break awkward silences by referencing the British TV advert about the posh drink Pimms.
Also used in football as a way of asking team-mates if you should shoot yet. (The answer being "Pimms o' clock!!!" or "not yet")
Occasionaly administers as a chat up line.
Jake: My Nan's just died...
Tony: Anyone for Pimms?
Tom: Hey Stuart, meet my friend John.
Stuart: Anyone for Pimms? (while smiling broadly and shaking his hand)
James: Anyone for Pimms? (while dribbling in a football match)
Dan: Pimms O' Clock!
(James shoots at goal)
(Ben sits alone at a bar and a girl sits next to him)
Ben: Anyone for Pimms?! (he shouts loudly at her)