v.1. To make a mistake, especially from nervousness or confusion: The runner scored because I messed up and dropped the ball.
2. To spoil something by making mistakes: I really messed up that chemistry test. That problem was difficult—I think I messed it up.
3. To cause someone or something to blunder or fail: I tried to pass the ball to the other player, but the loud shouting from the fans messed me up.
4. Slang To beat someone up; handle roughly: The bullies really messed up a couple of kids in a fight after school. If I told anyone what they did, they'd mess me up.
5. To cause something to be messy, tangled, or in disarray; muss something up: The kids messed up my hair, so I had to comb it all over again.
6. To injure, damage, or negatively interfere with something: Sitting in that uncomfortable chair really messed up my back. I messed my knee up when I fell down on the ice. Don't take those drugs; they will really mess you up.