knock the trough 

A phrase first used by a young man called William from Central Queensland, Australia in an incident involving a dyslexic message on a get well soon card.

Originally spelt 'knok the troth'

Now in common usage as a colourful way of saying 'tap that' meaning attempt to have some semblance of sexual relations

Person 1: Look at her/him, s/he's hot!
Person 2: Would you knock the trough?
Person 1: Oh yeah!

knock the trough by The Original Teeny September 26, 2006