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The Collaborative International Dictionary
Stupefacient

Stupefacient \Stu`pe*fa"cient\, a. [L. stupefaciens, p. pr. of stupefacere to stupefy; stupere to be stupefied + facere to make. Cf. Stupefy.] [Written also stupifacient.] Producing stupefaction; stupefactive. -- n. (Med.) Anything promoting stupefaction; a narcotic.

Wiktionary
stupefacient

a. (context medical English) inducing stupor or drowsiness (with a drug such as a narcotic or an anaesthetic) n. (context medical English) a drug that has the above effect