english
1
general::
Get in a fight, as in The driver got out and began to mix it up with the other driver. This expression uses mix in referring to physical mingling. [c. 1900]
American Heritage Idioms
2
general::
Phrase(s): mix it up (with someone) to fight with someone; to quarrel with someone. • Wilbur and Walt mixed it up for a while, and then things calmed down. • Richard came out of the shop and began to mix it up with Walt.
McGrawhill's American Idioms And Phrasal Verbs