Green’s Dictionary of Slang

1. a trouble-maker, a gossip, usu. deliberately malicious, one who ‘stirs things up’.

1989

J. Morton Lowspeak.

2. a fighter, a brawler.

1920		A. Baer Putting ’Em Over 10 Sept. [synd. col.] He was some mixer himself [...] grabbing off the Croix de Battle and a few other decorations .
1966		T. Keyes All Night Stand 53: A great mixer who could stick one on any three krauts when he was completely bevvied.

3. (US) one who socializes across racial lines.

1986

D. Barker Life in Jazz 149: Ward had a headstrong Italian girl. I had seen her around Harlem. There was a gang of them who would be with jazz musicians, white and black—they were mixers.

4. a fight, a brawl.

1999

J. Cameron Brown Bread in Wengen [ebook] Rameez and Afzal and Aftab in a proper mixer with security.