Green’s Dictionary of Slang

musher n.3

[? ext. of mush n.4 (2) or mush v.3 ]

a thug, a villain.

[UK]K. Sampson Outlaws (ms.) 12: Every musher in town wants to know the Brennan boys.
[UK]N. Griffiths Stump 52: Tommy said he’d chuck in another ton if we find that other musher.