Green’s Dictionary of Slang

(US tramp) a tramp who begs on a main street.

1915		Pittsburg Dly Post (PA) 20 Apr. 6/2: He describes himself as having been a ‘main stemmer’ (a tramp), a rounder, a barrelhouse bum and having traveled under the name of ‘Hoosier Slim’.
1927		‘Jargon of the Und.’ in DN V 464: stemmer, one who mooches the stem rather than batter the privates.
1931		Irwin Amer. Tramp and Und. Sl. 182: stemmer.–One who begs on the street.
1949		Monteleone Criminal Sl. (rev. edn).
1950		Goldin et al. DAUL 210/1: Stemmer. 1. (Hobo) A panhandler.
1962		Ragen & Finston World’s Toughest Prison 820: stemmer – One who begs on the street.