"Fess up or we'll all be caught"
to admit the truth, confess
Come on fess up. I know you aren't really that sick.
Also used as "fess it up"
1) To give or relinquish.
2) A request for booger.
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!
C'mon baby fess up some of that booger!
Dude, did you get her to fess it up last night?