"Fess up" is a colloquialism that means to admit or confess to something, especially something that one feels guilty about. There are several synonyms for this particular phrase, including "own up," "come clean," "confess," "acknowledge," and "reveal." All of these synonyms convey a sense of honesty and transparency, as well as a willingness to take responsibility for one's actions. Each word may differ slightly in connotation or emphasis, but they all point towards the same general concept of admitting to something that one has done wrong or has been keeping hidden. Overall, these synonyms provide a variety of alternative ways to express the idea of being truthful and upfront about one's actions.