From the OED on-line:

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skell, n.2

SECOND EDITION: ADDITIONS 1993

U.S. slang.

[Of uncertain origin; perh. shortening of skeleton.] 

  In New York, a homeless person or derelict, esp. one who sleeps in the subway system.
 
  1982 N.Y. Times Mag. 31 Jan. 21/3 Other New Yorkers live there [sc. the subway]..eating yesterday's bagels and sleeping on benches. The police in New York call such people ‘skells’. Ibid., These ‘skells’ are not merely down and out. Many are insane, chucked out of New York hospitals. 1988 Newsday (N.Y.) 22 Feb. 6 The delirious, crazy people whom cops call ‘skells’, the down-and-outs, the grungy and hopeless, garbage-heads who use any foreign substance known to man to alter reality.