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. A song to quiet babes or lull them to sleep; that which quiets. 2. Hence: Good night; good-by.[Obs.]