Onomatopoeia......what?!?

The first question out of Elliott's mouth this afternoon when he got off the bus was this.......

Elliott:   "Mom, do you know what onomatopoeia means?"
Me:  "What means?"
Elliott:  "Onomatopoeia."
Me:  "What?"
Elliott:  "Onomatopoeia!"
Me:  "Elliott, I can't even say the word, how am I supposed to know what it means!"
Elliott:  "I can't believe you don't know!  It's a sound that's a word!  Like boom! Or bang!"
Me:  "Oh."  (Outsmarted again.)

I think I vaguely remember hearing that word in school, but I know for sure it wasn't in kindergarten!