Wiktionary
fess up
vb. (context intransitive slang English) To confess to something; to admit something.
Usage examples of "fess up".
Butch Rowden over at the meat market would fess up to it, I guess - he keeps a picture of one of the cars they had up on the wall where he cuts meat.
Once the arsonist found out about the killings, he wasn't willing to 'fess up to any part in it.
Can't you 'fess up just a little and help us clear things up a mite?
If he was somebody she was supposed to know, then, damn it, let him fess up.