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1.
A momentary increase level of excitement between your legs
OMG, did you see him/her...what a flash in the pants....
by SOB13 September 25, 2009
 
2.
a momentary burst of sexual arousal (a pun on flash in the pan)
I had a flash in the pants from thinking about the night before, but realized I can't let it interrupt my work.
by The Return of Light Joker November 01, 2011