When playing pool (english) if the black is sunk whilst one player still has all seven of his/her balls on the table, he/she must play the next game naked.
(some play so you only drop your trousers and underwear, or for girls, take your top and bra off)
see also six balled
and five balled
"HAH! i seven balled you! get your clothes off sucker!"
"oh man, he seven balled me yesterday, everyone saw my cock
"lol you got pwnt