Hands of jazz

Wearing a sling on his injured arm has had its drawbacks for Sam.

As we were all in the kitchen getting ready for dinner, Sam said "Oh! you know what happened to me today?"

"No, what?"

"In school we had a performer come. At the end the lady called on me because she thought I had a question."

"Why did she think that?"

"Because I did jazz hands."

"What?" [laughing]

"Well everyone clapped at the end and I didn't want to be rude but since I only have one arm I can use, I just kinda shook one hand in the air. You know, jazz hands. It was so embarrassing. She thought I had an important question or something."

When did Sam learn the term "jazz hands"?
{We do lightly make fun of jazz hands now and then in my family...
maybe he overheard this?}
Why does this make me laugh so hard?
{Yes, it is the gesture for clapping in sign language...
does he think instead that they are jazz handing?}