Green’s Dictionary of Slang

sag v.¹

[? fig. use of SE sag]

(US tramp) to beat with a truncheon.

1927		‘Jargon of the Und.’ in DN V 461: Sag, To club senseless.
1931		Irwin Amer. Tramp and Und. Sl. 163: SAG.–To beat, as with a club.