"Fly in the teeth of" is an idiom that means to act in opposition or defiance of something. Synonyms for this phrase include "defy," "oppose," "contradict," "disobey," "rebel against," "challenge," or "go against." These terms all convey the idea of acting in a way that is contrary to a given law, policy, or standard. When people "fly in the teeth of" something, they may be taking a risk or challenging authority in some way. These synonyms help to highlight the various ways in which someone can act in opposition, and demonstrate the importance of standing up against unfair or unjust actions.