When you give someone rough anal sex that makes they're eyes water like they have been peeling onions.
I gave Mandy a dirty onion last night.
by blongly January 07, 2008
When someone gets fucked up the ass and then proceeds to suck off the same person who has just done them up the bum.
Last night Kate got did a dirty onion.
by jsizz July 31, 2006