Green’s Dictionary of Slang

violence, esp. when allied to robbery or extortion.

1937		‘Boxcar Bertha’ Sister of the Road (1975) 263: We were going strong-arming in a house over on the North Side and the police got wise and pinched me.
1942		N. Algren Never Come Morning (1988) 18: Bruno didn’t do any strongarming for the fun of it.
1949		N. Algren Man with the Golden Arm 117: The worst thing the neighborhood bucks got pinched for was strongarming.