The phrase "put up a front" is often used to describe someone who is pretending to be a certain way in order to hide their true feelings or intentions. Antonyms for this phrase could include "be transparent," "show vulnerability," "be authentic," "be genuine," or "reveal your true self." Instead of trying to conceal their emotions or motives, someone who is being transparent is open and honest about their thoughts and feelings. Showing vulnerability can also be seen as an opposite of "putting up a front," as it involves being open about one's weaknesses and flaws. Ultimately, the goal of being authentic and genuine is to show others your true self, without hiding behind a facade.