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1.
A boner that arises immediately from the presence of a really hot chick or some female body parts on serious display.
"Jessica walked in wearing a thong and a push-up bra and immediately I had a flash-boner."
by Dan_07 October 26, 2011
 
2.
an erection that last under 1 min.
bro 1: dude was that ur mom

bro2: yea

bro1: ok because ur sister just gave me a flash boner.
by Erin Mcracken November 13, 2009