The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]