rock-a-bye

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.
by Rockbabe! January 05, 2018
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Rock-a-bye Baby

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.
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