Where did this come from?

A conversation from today:

Saw the Super Six Year Old: Mom, how do you spell bosom?

Me: (Caught off guard) Why do you want to know?

Saw the Super Six Year Old: Is there a Z in it? Is it b-u-z-z-u-m-z?

Me: Noooo, um it's b-o-s-o-m. When did you hear this? Who was talking about bosoms?

Saw the Super Six Year Old: What are you so grumpy about?

Me: I'm not grumpy, I just want to know who was talking about this with you.

Saw the Super Six Year Old: It's Grandma, in the booook (exasperated face).

Me: What book?

Saw the Super Six Year Old: WHAAAAATTTT ARRRRE YOUUUU SOOO GRUMPYEEEEE AAABBOUUUUTTT (thinks to himself - did Mom bump her head?) You know, the kids get lost in there.

Me: OOOOHHH, yea, What Are You So Grumpy About. Ok, now I know what is going on. The book I have read to you a hundred times (did I bump my head?).

Saw the Super Six Year Old: Yea, so b-o-s-o-m. Are you sure there isn't a Z?

Me: Yup, I'm sure there's no Z in bosom.

Ms. P: (just breezing through the room) Bosom is another word for boobs!

I need a nap.