Green’s Dictionary of Slang

1. (drugs) phenobarbitol, a depressant.

1953		J.L. Herlihy ‘Sleep of Baby Filbertson’ in Sleep of Baby Filbertson and Other Stories (1964) 12: It was like that quick and sensuous falling into sleep on a night when he’d stolen one of the magic pheenies.
1971		E.E. Landy Underground Dict. (1972).
1977		(con. 1960s) Nicholson & Smith Spend, Spend, Spend (1978) 192: He’s took phenals.
1981		D.E. Miller Bk of Jargon 338: phennies: Seconals.
2001		ONDCP Street Terms 17: Phennies — Depressants.

2. attrib. use of sense 1.

1953

J.L. Herlihy ‘Sleep of Baby Filbertson’ in Sleep of Baby Filbertson and Other Stories (1964) 21: He wanted desperately to get away from it, perhaps into a pheeny sleep.