When a man and a woman have anal sex and the woman proceeds to excrete onto the mans penis, the women then sucks the excretion off of the mans penis...
Boy: "Hey babe, can you give me a rimjob?"
Girl: "But you had a burrito about an hour ago."
Boy: "I promise I won't fart."
Girl: "Fine."
*Proceeds to start rimjob*
*Boy farts... loudly...*
*Girl looks up*
Girl: "I can't believe I got talked into a grimjob."