The Collaborative International Dictionary of English v.0.48:

Soporiferous \Sop`o*rif"er*ous\, a. [L. soparifer; sopor a heavy
   sleep + ferere to bring.]
   Causing sleep; somniferous; soporific. "Soporiferous
   medicine." --Swift.
   [1913 Webster] -- Sop`o*rif"er*ous*ly, adv. --
   Sop`o*rif"er*ous*ness, n.
   [1913 Webster]