It’s about time. We are now on Instagram!
1977, from sick (adj.) in the mental sense + ending as in weirdo. Sickie "a pervert" is attested from 1972, and cf. sicknik (1959) "pervert, obscene comedian" (applied to Lenny Bruce), with fad ending -nik.
A mentally unhealthy person; a psychopath: It was not considered very kosher to toss out the sickies once they had taken their final vows/ Only sickies get involved with married women (1973+)