Green’s Dictionary of Slang

1. a heroin addict.

1966		Richard Goldstein One in Seven: Drugs on Campus in Lingeman (1969) n.p.: For students who use heroin are referred to as sickies by even the most Bohemian students.
1969		R.R. Lingeman Drugs from A to Z (1970) 229: sickie [...] A college student who uses narcotics.

2. (US campus) a homosexual.

1967–8

Baker et al. CUSS 197: Sicky A homosexual.

3. (orig. US, also sickee) anyone considered to be ‘sick in the head’, insane, crazy [sick adj.¹ (4)].

1968		K. Brasselle Cannibals 215: He’s a twenty-four-carat sickee, believe me.
1977		C. McFadden Serial 21: Screaming at Harvey about how he was a ‘sickie on a power trip’.
1986		S. King It (1987) 41: I’d feel better if I thought it was just one person, one isolated sickie.
1987		‘Joe Bob Briggs’ Joe Bob Goes to the Drive-In 132: I am not a psycho, sicky or anything else beside a normal High School student.
1994		N. McCall Makes Me Wanna Holler (1995) 179: He was a sickie with a murderous heart.
2000		Indep. on Sun. Rev. 18 June 65: Not a good word, that; iss like callin’ sickies who fuck with kiddies ‘child–lovers’, hey?
2014		B. Kroeber ‘Be My Alibi’ in ThugLit Sept. [ebook] They were sickies, those two.