Green’s Dictionary of Slang

(US Und.) robbery of itinerant workers at gunpoint; thus cat-up man, one who commits such robberies; also as v.

1931		Irwin Amer. Tramp and Und. Sl. 49: Catting Up. – Robbing ‘cats,’ which see. This was often, and is now occasionally, done by criminal tramps, yeggs or jungle buzzards who enter a freight car with the unaware ‘cats’ and make them disgorge at a pistol’s point. Often the victims are made to jump from the moving train after the robbery.
1935		A.J. Pollock Und. Speaks 20/1: Cat-up, a robbery.
1949		Monteleone Criminal Sl. (rev. edn) 45: cat-up The robbery of an itinerant worker [...] cat-up man A robber of itinerant workers.