Green’s Dictionary of Slang

1. to pick a fight; thus to assault.

1906		Eve. Star (Washington, DC) 30 Sept. 39/2: ‘He’d say things that would make me want to muss him.’ ‘Precisely what he needs — to be mussed,’ says Morris.
1949		Monteleone Criminal Sl. (rev. edn).

2. to argue.

1917

F. Packard Adventures of Jimmie Dale (1918) I iv: Say, he mussed all over the place, and went and fainted like a girl.