1. To fight or argue with someone. That guy's always coming in here and trying to mix it up with the other patrons. When it comes to politics, Jerry sure likes to mix it with those who don't share his views.

2. To stir or fully combine the thing being discussed. I added all the ingredients to the bowl. Can you mix it up?

3. To confuse something for something else. I thought you ordered diet. Or did I mix it up?

4. To jumble something that it is no longer in proper order. I gave you that file in a specific order. I hope you didn't mix it up. I'll give you the deck of cards, and you mix it up.

5. To compete or interact with other people, especially those that are older, stronger, or more talented than one is. He's only 17, but he's so talented on the football field that he's been mixing it up with the college team this year. I have felt like a fraud trying to mix it with real professionals ever since I joined this law firm last fall.