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1.
A moment while having sexual intercourse when you stop for a second or 2 in penetrating the woman's (or if gay, man's Asshole) vagina.
No Fuck Lag:
===(>)

Fuck Lag:
===> ()
by Kyle Keith May 17, 2006
 
2.
The result of fucking so much and to the point that one is physically and mentally exhausted from the process that they need an ample amount of sleep to regain energy and thus being able to perform daily tasks, along with more fucking. It is similar to being jet-lagged, but from fucking too much.
Blake: Dude, Ashley and I fucked for 8 hours last night, I'm exhausted.

Pierce: You must have a seriously bad case of fucklag.
by GlazeCraze June 11, 2009