Source : Webster's Revised Unabridged Dictionary (1913)

lullaby \lull"a*by\, n. [From {Lull}, v. t. ]
   1. A song to quiet babes or lull them to sleep; that which
      quiets. --Shak.

   2. Hence: Good night; good-by. [Obs.] --Shak.

Source : WordNet®

lullaby
     n 1: a quiet song intended to lull a child to sleep [syn: {cradlesong},
           {berceuse}]
     2: a quiet song that lulls a child to sleep [syn: {cradlesong}]
     [also: {lullabied}]