When the presence of a girlfriend (either real of fictitious) is introduced by a male during a conversation with a female it is considered dropping a "girlfriend bomb." This is typically done to deter further or perceived pursuit of the male by the female. Either the guy finds the girl he's chatting with unattractive and fears she may be interested in him. Or he's intimidated by her. Thus, he needs to make her aware of "competition", that's he's desirable and taken or that he is *NOT* desperate.
It's basically saying "Hey look...someone likes me OTHER than you!"
(girl and guy casually chatting about nothing...)
Girl: yeah, so that's when i decided i wanted to see Iron Man.
Guy: *clears throat* um yea me and my girlfriend went to see that last night.
girl: wow...you dropped a girlfriend bomb..how lame.
36π 6π
When a male you are interested in casually reveals that she has a girlfriend (intentionally or otherwise).
I really liked this guy, but he dropped the girlfriend bomb
64π 27π
When a male you are casually conversing with mentions within the first five seconds that he has a girlfriend. Not only were your intentions completely civil, but you realize that he is either extremely turned off by you that he feels the need to compensate his repugnance with his martial status, or that his pre-pubescent feelings have gotten the best of him because βsexkitten10β instant messaged him last night.
I just liked this guy's sweet Nike's so I asked him where he bought them, sadly he dropped the girlfriend bomb on me because he couldn't handle my sex appeal.
22π 11π
When a female friend (or otherwise) askes you out and you tell them you have a girlfriend
Johnny dropped the Girlfriend bomb on that fucking gold digger
4π 4π