(verb) To solicit conversation with another man while standing at a urinal, on any pretext, not necessarily sexual. For instance, friendly banter in a public house toilet or 'talking shop' in workplace bathroom. Usually an unwanted intrusion while passing water.
"Hey Dave, do you need extra time on that Kenton report?"
"Yeah, I discussed it with Marty - I just Rhubarbed him in the bathroom. His wife is a lucky woman"
A punishment administered to boys in British Secondary schools (now uncommon since corporal punishment outlawed in 1988) whereby the miscreant is called to the front of the class. There he is placed supine across the teachers knee, his trousers and knickers are pulled down below the calves and his buttocks are slapped as hard as possible with the open hand between 6-8 times. The recipient is then rolled over and receives a violent punch to the bell-end or scrotum with the clenched fist.
Usually carried out to re-inforce principles/basics when teaching quadratic algerbra.
Word, blud - you check Snoopi put glass n sheeat in Mr Campbell's tea?
Yea, dog! An got his batty messup widda Monkey-Spank! (said by a white kid)