1. To confuse or mistake one person for another. I always mix Kelly up with her younger sister—they look so much alike! Sorry, I think I'm mixing him up with someone from the accounting team.

2. To involve or embroil one with some other person, especially someone who is problematic, unpleasant, dangerous, etc. Often used in passive constructions. I heard Tom's gotten mixed up with some pretty shady people since he moved to New York City. I don't want you mixing Sarah up with any troublemakers, you hear me?