The point at which the number of drinks required to make a person sexually attractive enough for intercourse exceeds the point to which the man can maintain an erection.
Wingman: Check out that girl at the bar. Whadda think?
Man: 5 drinks. Maybe.
Wingman: Alright well how about the chick in the blue dress checking us out over there?
Man: 7, doubtful.
Wingman: And the girl dancing by herself to Superfreak on the dancefloor?
Man: Dude, that's an Event Horizon right there... No way.