look up any word, like dirty sanchez:
 
1.
Quickie Fast Sex Real Fast
The girl that I was afraid to ask out sneaked up behind gave me a surprise flash fuck.

Before the Ayatollahs shut down the nightclubs, many an Iranian enjoyed a flash fuck.
by Ali Bashir January 31, 2006
 
2.
1. Sex so fast that you don't even realize it's over when she's already dressed and leaving.

2. A flashbang grenade.

"That wasn't even 5 mintues"
*door slams, man looks around confused*
"Was that a flashfuck?"
by Spartan A007 January 15, 2009