A "BONER KILLER" is when something good is about to, or is happening and all of a sudden it turns out to be bad. Its another way to express your hopes being let down.
" I went to KFC and when i was in line, I saw a roach run across the floor. That was such a BONER KILLER.So I left."
Example 1: You meet a cute girl/guy. They stink.You all of a sudden loose all attraction.
Example 2: Your at a party and your having fun. Then some idiot decides to start a fight. It gets out of control, the cops come over, and you get arrested. Your night is ruined.
Example 3: You have a crush on someone and then you come to find out something terrible about them (i.e. STD, hoe, player, too many kids, married). Ruins the image you had in your head about them.