the exact opposite of a hail mary which is a miraculous success. a "fail" mary is the extreme degree of an epic fail. the worst possible type of fail one could ever experience.
Person 1: "Did you see what happened during the last 10 seconds of the game last night?"
Person 2: "Yeah...the receiver jumped for the ball, but missed it completely..I mean..it just went right above his head."
Person 1: "You'd think the quarterback went for the "fail mary" play."