A Mongolian Breakfast is a sexual act where the male (or lesbian partner) defecates on the Vagina of the receiver and then proceeds to eat out the receiver, defecation and all.
"Jeff had a Mongolian breakfast with Caitlin this morning"
Jeff: Hey I'm Hungry
Caitlin: well do you like Mongolian food
Jeff: only if its breakfast
Caitlin: Well lets have a Mongolian Breakfast