Use the adjective unbecoming when a person does something that's awkward or inappropriate, like using foul language at your grandmother's tea party.
Sometimes the word unbecoming means "unattractive," as in, "I loved the light blue dress, but I thought the yellow dress was rather unbecoming." Behavior that is unattractive, especially in a particular setting or with certain people, can also be unbecoming. It's unbecoming to burp loudly at the opera. If it's impolite or unseemly, it's unbecoming. The word stems from becoming, which means attractive or tasteful.