D defs.my
Entry 2 senses Webster, 1913

lullaby

/lŭl'-əb-īˌ/ · lull·a·by · IPA /ˈlʌləbaɪ/
01 n. A song to quiet babes or lull them to sleep; that which quiets.
  1. 1.
    A song to quiet babes or lull them to sleep; that which quiets.
  2. 2.
    Hence: Good night; good-by.[Obs.]