yes, i suppose it IS a question word.

Eliot: Mommy? Is "howcumum" a question word?
Me: "Howcumum"? I've never even heard of that word. What does it mean?
Eliot: You don't know what "howcumum" means?? *rolls eyes* It's like if you say to somebody.. "How come, uhm... the sky is blue?"