Green’s Dictionary of Slang

(drugs) a Seconal capsule.

1972		E. Folb Urban Black Argot 137: F-40s 1½ grains of ‘Seconal’ (so called because the Eli Lilly drug company stamps this particular barbiturate with the identifying letter and number ‘F40’).
1980		E. Folb Runnin’ Down Some Lines 168: Other terms associated Seconal with its commercial identification number (and potency) – F-40s, Lilly F-40s, F-60s, F-66s. [Ibid.] 177: Like them little ol’ ’40s, they will set you on your ass and have your mind in a daze.