Green’s Dictionary of Slang

(US Und.) to drill a safe as part of breaking it open.

1928		J. Callahan Man’s Grim Justice 53: I was never so happy in my life as I was the night I ‘stemmed’ my first ‘pete.’.
1950		Goldin et al. DAUL 209/2: Stem in. To drill a hole in the door of a safe before inserting explosives or forcing the combination; to drill an opening for the insertion of a can-opener.