He doesn't have one. However, the key is tiny.INVENTORYTiny Key x1
No way, that would hurt.INVENTORYTiny Key x1Fact: The keyhole on Capt. Obivous' chest is tiny, the tiny key should fit.
He does not have a butt, instead he pokes it with the big key. Obivously, there are no traps.
He opens the chest and gets The Zero Divider! However, on one of its sides you read: '50% CHANCE OF DIVIDING BY ZERO'INVENTORYZero Divider