Noun: "Hello my little kabookaboom!"
Verb: "Oops! Did you kabookaboom?"
Exclamation: "Kabookaboom!" - while tickling, etc.
No idea where this word came from, but both Mike and I have used it since Ben's birth. It's Ben's nickname, a word to distract him from being upset or to get his attention, a synonym for falling, bonking his head, and in place of peekaboo.