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1. To tinker or fiddle with something. Hey, don't mess about with the thermostat—it needs to stay at 68 degrees. Grandpa's still messing about with that old radio, but I doubt he'll get it to work again.
2. To test or use something in order to learn more about it. I'm messing about with a different platform for my blog, and I can't decide if I like it or not. After messing about with it for a while, I finally got the hang of the controls.
3. To engage in aimless recreation or frivolous time-wasting; to fool about. Quit messing about and work on your book report already, Scotty! It's no wonder we're falling behind schedule, with the amount you lot mess about in the office!
4. To tease or play a prank on one, often by trying to get them to believe something that is not true. In this usage, a noun or pronoun can be used between "mess" and "about." Come on, stop messing about. There's no such thing as ghosts. Don't mess your brother about like that—you know he's got an overactive imagination.
5. To make something disorderly or untidy. In this usage, a noun or pronoun is used either before or after "about." Hey, I just got my hair styled—don't mess it about like that! Someone's been messing about the papers in my office, and I want to know who is responsible!
(with something) Go to mess around (with something).