A fork of Rural Dictionary
It's an interjection, always used to settle a baby or child down to sleep.
"rock-a-bye, Baby," sang Alfred in strident tones and he swung the child high in his arms.
Is a nursery rhyme and lullaby.
The most common version used today is: Rock-a-bye baby, in the treetop, When the wind blows, the cradle will rock, When the bough breaks, the cradle will fall, And down will come baby, cradle and all.