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2.
Magic sense to know when you're about to get caught having a wank.

Be alert, gentlemen.
Man, I almost got caught fapping by my mom, but then deja fap kicked in, and I covered up just in time.
by anarchomaniac January 17, 2011
 
1.
When surfing pornography and seeing a video you recognize from years before and begin furiously beating it in response.
Dude, I saw this video last night that I hadn't seen in years... totally deja fapped it...
by David Duchovny Woods May 04, 2011