Green’s Dictionary of Slang

(US prison) to have a fistfight.

2000		Other Side of the Wall: Prisoner’s Dict. July 🌐 Fop Fops: To fight with fists.
2006		Prison Slang Mommyblogger mydogharriet.blogspot.com 2 Mar. 🌐 You better be prepared to put up your fop-fops and duff that rustler out.