GBK is an adjective used to describe a man or women or situatuion that killed your girl boner.
A "GIRL 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.
Someone that is so ugly and repulsive that when they walk into the room any girl who had an A Girl Boner immediately goes flacid.
Example 2 : Yo that Dude looks like a zombie, he is such a GBK!
Example 3 : "Yo , enjoying the party?"
"Yea man!! Its fucking tight.. I ran over a dog on the way here" "Dude... Why you got to be such a GIRl BONER KILLER!!!"
Example 4 : You meet a cute girl/guy. They stink.You all of a sudden loose all attraction.
Example 5 : 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.
Example 6 :When your Girl boner is pretty much killed. Can happen is a lot of ways, some guys are pro also known as a GBK
Matt- "I'm going home"
Holly- "WHAT A FUCKING GBK (girl boner killer)! Well, have fun beating your meat bitch, I'll just stay here and Flick my bean ! "