Green’s Dictionary of Slang

1. (also stiffie) a corpse.

1916		J. Hargrave At Suvla Bay Ch. xi: The smell of human blood soaking its way into the sand from those two ‘stiffies’ on the beach.
1930		W.R. Burnett Iron Man 32: You’d think I was a stiffy stretched out like this.
1986		C. Stroud Close Pursuit (1988) 82: Haul ass over to mama’s and get something definite going about your little stiffie this morning.

2. (US tramp) a tramp who poses as being paralyzed.

1917		‘A-No. 1’ From Coast to Coast with Jack London 22: He introduced himself as ‘Stiffy Brandon.’ His moniker indicated that for a beggar craft he had chosen the one which imposed upon the credulous by stimulating the awful affliction of the paralytic.
1923		N. Anderson Hobo 100: [From A No. 1, The Famous Tramp] 4. Stiffy. Simulated paralysis. 5. Dummy. Pretends to be deaf and dumb.
1927		in ‘Jargon of the Und.’ in DN V.

3. (S.Afr.) a paralytic.

1963

L.F. Freed Crime in S. Afr. 106: A ‘stiffy’ is a paralytic.