The phrase "pat back" typically means to offer praise or encouragement to someone. However, there are several antonyms for this phrase, which could indicate the opposite sentiment. For example, "criticize" would be an antonym of "pat back," meaning to point out faults or flaws. Another antonym could be "discourage," indicating that someone is not providing positive feedback or support. Additionally, "condemn" could be an antonym for "pat back," suggesting that someone is disapproving or expressing strong disapproval. These antonyms highlight the importance of offering constructive feedback and criticism, rather than just blindly praising someone.