Source : Webster's Revised Unabridged Dictionary (1913)

Placebo \Pla*ce"bo\, n. [L., I shall please, fut. of placere to
   please.]
   1. (R. C. Ch.) The first antiphon of the vespers for the
      dead.

   2. (Med.) A prescription intended to humor or satisfy.

   {To sing placebo}, to agree with one in his opinion; to be
      complaisant to. --Chaucer.