You have to take the whole line into consideration if you want to understand it :
There are 2 possible origins that I know of, the American & the English ... and I quote ...
Origins of words to "Rock a bye baby" in American history...
The words & lyrics to the "Rock A Bye Baby" rhyme are reputed to reflect the observations of a young pilgrim boy in America who had seen Native Indian mothers suspend a birch bark cradle from the branches of a tree. Thus enabling the wind to rock the cradle and the child to sleep.
I am not sure 100% of this explanation but I`ll search for some more explanations. If I find something more interesting I`ll let you know.
Hush a bye baby nursery rhyme lyrics with origins and history