Fess (verb)

1. To make a formal acknowledgement or confession of (wrongdoing or guilt).
2. To admit or acknowledge something as true or real.

Origin:

Early 16th century (in the sense "make a formal acknowledgement or confession of (wrongdoing)"): from old french confesser, from latin confessus, past participle of confiteri "acknowledge, confess".

Examples:

1. He was asked to fess up to his mistake.
2. She was forced to fess up to her part in the crime.
3. He finally decided to fess up and tell the truth.
4. I have to fess up that I don't know the answer.
5. He was caught in a lie and had to fess up.