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1.
When an Irish man has sexual intercourse with a woman until he is about to cum, and then stops. The female then clinches her vagina to make the mans penis explode.

No longer only for the Irish, but received it name as it was traditionally preformed by Irish woman who no longer wanted children from their husband.

patty- Irish
mayonnaise- Because that is all that is left afterwards.
My wife said she doesn't want any more children, I am afraid if i do not get a vasectomy she will give me a patty mayonnaise.
by Loss Prevention Officer August 02, 2011
 
2.
A very flabulous release of gases out of the vagina. Characterized by the trademark wet sound of flapping lips and sour mayonnaise-like smell.
When Doug Funnie finally gets to go down on Patty Mayonnaise.
by Agent Kaniff June 22, 2007