Irrumatio is the act of thrusting of the penis into the mouth of a partner, or between the thighs (also known as intercrural sex) or more rarely between the abdomens of two men. In the ancient Roman sexual vocabulary, irrumatio is strictly a form of os impurum, oral rape, in which a man forces his penis into someone else's mouth, almost always that of another man.
Learn more about irrumatio from Wikipedia.
to put it bluntly, the act of face fucking
i was having irrumatio last night, its even better then fellatio!
Source: Urban Dictionary