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1.
to confess
"Fess up or we'll all be caught"
by Pete January 20, 2004
 
2.
to admit the truth, confess
Come on fess up. I know you aren't really that sick.
by The Return of Light Joker November 13, 2009
 
3.
Also used as "fess it up"
1) To give or relinquish.
2) A request for booger.
1.
Person A: I'm goin' to the store - you want something?
Person B: Yeah, get me a dope and a moon pie.
Person A: Fess up some $, biatch!

2.
C'mon baby fess up some of that booger!

3.
Dude, did you get her to fess it up last night?
by [KmA] Jumper May 25, 2005