Mental Floss magazine recently reprinted a 2004 article titled 10 Latin Phrases You Pretend to Understand. I plagiarize shamelessly.
persona non grata – an unacceptable person
Remember your old college buddy, the one everybody called Chugger? Now picture him at a debutante ball, and you’ll start to get a sense of someone with persona non grata status. The term is most commonly used in diplomatic circles to indicate that a person is unwelcome due to ideological differences or a breach of trust. Sometimes, the tag refers to a pariah, a ne’er-do-well, a killjoy, or an interloper, but it’s always subjective.