Green’s Dictionary of Slang

ham n.3

[backform. f. hamfisted, plus shape and mis-pron]

(US) the hand.

[US]Wash. Post 10 Dec. 4/4: Slimmy was leanin’ on the mahogany wettin’ his nose [...] when this gee blows in with a rod in his ham.
[US]‘F. Bonnamy’ A Rope of Sand (1947) 100: Hjerleid built most of it with his own two hams of hands.