Fess up
Definition 1 of 1
Pro Tip 1/3Informal Tone
Used mostly in casual conversations, not for formal situations like business meetings or official documents. 
You need to fess up to eating the cookies; Mom won't be mad.
Pro Tip 2/3Slight Urgency
Implies a sense of urgency or a pressing need to reveal the truth. 
Come on, fess up now before things get worse.
Pro Tip 3/3Friendly Connotation
May sound friendly or coaxing, less harsh than a direct command. 
I know you're hiding something. Fess up!