Green’s Dictionary of Slang

1. one who is mentally impaired.

1931		G. Milburn ‘Convicts’ Jargon’ in AS VI:6 438: feeblo, n. A prison intelligence test; a simple-minded person.
1946		W.L. Gresham Nightmare Alley (1947) 81: If I can’t read a Bible-spouting, whoremongering, big-knuckled hypocrite of a church deacon [...] I’m a feeblo.

2. a drug addict.

1949		Monteleone Criminal Sl. (rev. edn).
1953		Anslinger & Tompkins Traffic In Narcotics 308: feeblo. A drug addict.