It's when your date's gone really well, you've made all the right moves, she is digging you, you go back your place and you fail to seal the deal due to a number of reasons including:
A.) unfortunate accident (ie: insult her/her mother)
B.) Whiskey dick
C.) she finds out you don't actually live in a penthouse, rather just subscribe to Penthosue
D.) She turns out to be he

Expression origin: It comes from the Cleveland Indian's "closer" Joe Borowski. He has the best guys in the game behind him but teeters on destruction every single night.