Turntail is a derogatory term used to describe a person who abruptly changes their opinion or behavior in order to avoid responsibility or confrontation. Some synonyms for turntail include coward, deserter, traitor, backstabber, and renegade. You might also use phrases like "faint-hearted," "craven," or "yellow-bellied" if you want to emphasize the person's lack of courage or sincerity. Alternatively, you could use figurative language to describe the person's actions, such as saying they "swerved off course" or "went AWOL." Ultimately, the choice of synonym will depend on the context and degree of the person's betrayal or cowardice.