Moms-a-butt

I was trying to use my pump this morning and discovered that it needs new batteries. Before I thought about it, I said "What a pain in the butt". My toddler heard me and went running through the room singing loudly "Mom's-a-butt! Mom's-a-butt! Mom's-a-butt!" over and over again. I couldn't help it and had to laugh. It was especially funny because she doesn't know what the word means.