rock-a-bye also rock·a·bye or rock·a·by (r?k??-b??)interj. Used to lull an infant or child to sleep.[rock + (lull)aby.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.