When, usually at the bar or a party, a guy (your buddy, unless your a total dick) goes for a broad, but you call out something that won't give him a chance. It's usually about an STD or him having a wife.
From, "You gotta slay a few dragons to get to the princess" a dragon slayer is someone who is out doing dirties with the hopes of finding a nice girl to really date. Or he's just using it as an excuse for never settling down.
"Gillies has been a dragon slayer longer than Jesus!"
"He'll never get to a princess"
"No way does he even want to!"