Rock-a-bye baby

Question

Somebody told me that Rock-a-bye baby is a nursery rhyme. But how do I use the phrase actually and what does it mean? This is the lyrics from the nursery rhyme:

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

As a non-native English speaker, it's hard for me to interpret the meaning. This phrase also can be heard in the Eminem song When I'm gone.

That's Slim Shady, yeah baby Slim Shady's crazy
Shady made me
But tonight, Shady's rock-a-bye baby.

Dictionary.com says it's an interjection that means "used to settle baby to sleep". Then what does it mean? And how to use it?

Answer 1

Rock-a-bye has no particular meaning, though it's obviously patterned on lullaby, with rock replacing lull. Light songs and verses often incorporate nonce-expressions (expressions made up for the immediate occasion) and nonsense words and intruded syllables. They're intended not to communicate a 'meaning' but to create a rhythm or a mood or to establish a sonic pattern.

... be you blithe and bonny
Converting all your sounds of woe
Into Hey nonny, nonny —Shakespeare

Ob-la-di, ob-la-da, bra-la-la how the life goes on —The Beatles

They're particularly common in songs and verses for very young children:

Hey-diddle-diddle, the cat and the fiddle
The cow jumped over the moon

Once they've been created, however, these nonsense patterns take on a life of their own (the word lullaby itself probably started as a series of meaningless but soothing syllables), and it's not at all unusual for them to be employed as allusions to the work or genre from which they're drawn. Cole Porter, for instance, in his reworking of Shakespeare's Shrew as Kiss Me Kate, echoes hey, nonny, nonny very wittily:

With a hunny, nunny, nunny
And a hey, hey, hey
Not to mention money, money
For a rainy day
I come to wive it wealthily in Padua

In 1918 Al Jolson had a #1 hit with the song Rock-a-Bye Your Baby with a Dixie Melody, where rock-a-bye has taken on the sense "sing a lullaby to". And Eminem is doing something similar in When I'm gone. The song is about the conflict between the demands of his career and the needs of his daughter and estranged wife. The couplet you cite expresses this in the contrast between the two rhyming terms: "Slim Shady" is the aggressive persona he adopts when he sings to his audience, the persona which "made" him a star, "Rock-a-bye-baby" is the persona he strains to adopt at home, singing to his daughter.

Answer 2

This phrase, to my knowledge, doesn't really have a very specific meaning. In fact, I am certain that when most people hear it, they think of the nursery rhyme and the implications of putting a young child to bed.

I just checked the OED, this is often used to start various nursery rhymes starting in 1805.

As far as usage goes, this isn't really something that you would use in day-to-day conversation. Both of the examples above are taken from types of songs; accordingly, it is difficult to give them meaning in the same way that we give a word meaning in a normal sentence--and I think the second example is a direct reference to the first.