1. To change, fiddle, or play with something, often without permission to do so. My tools are not toys—please do not mess with them. Hey, don't mess with the thermostat—it needs to stay at 65 degrees.

2. To clash with someone or something. That group of boys is dangerous—don't mess with them.

3. To tease or mock someone. Sarah's a very sensitive kid, so don't mess with her.

4. To confuse or puzzle someone. One minute he's flirting with me, and the next, he's completely disinterested. His mixed signals are totally messing with my head!