1. To cause great psychological harm to someone. A noun or pronoun is used between "mess" and "over." Wow, that movie really messed me over—it was so bleak and depressing! I heard that someone spiked his drink with LSD, and it really messed him over.

2. To deliberately attempt to deceive, manipulate, or misinform someone in order to exploit them. A noun or pronoun can be used between "mess" and "over." The company has been accused of messing over its customers with a change to the way it processes payments, resulting in hundreds of thousands of incorrect deductions from their accounts. I can't believe he would mess me over like that—I trusted him!