faux pas
An embarrassing social mistake that breaks polite behavior rules.
A faux pas (pronounced “foe pah”) is an embarrassing mistake in a social situation, especially one that shows you don't know the proper way to behave. The term comes from French and literally means “false step,” like tripping while walking.
When you commit a faux pas, you've accidentally done something awkward or inappropriate. Showing up to a formal dinner in shorts and a T-shirt would be a faux pas. So would loudly interrupting someone who's giving a speech, or forgetting to thank someone who gave you a gift. These aren't terrible crimes, but they're mistakes that can make everyone uncomfortable.
What makes something a faux pas is that it breaks unwritten social rules. You might not realize you've made one until you see people's reactions. Maybe you accidentally called your teacher by their first name, or you wore your baseball cap indoors at a fancy restaurant. The key is that a faux pas usually comes from not knowing better rather than trying to be rude.
Everyone makes a faux pas sometimes. People often apologize sincerely if they realize their mistake, learn from it, and move forward. People who can laugh at their own faux pas and learn from them tend to become more socially confident over time.