To plume up my will in double knavery

I know exactly one word in American Sign Language.

Guess what it is.

EDIT: I really hope the first guess is right.

There was a little boy by the name of Billy. Billy was an ordinary little boy who did ordinary little boy things, like playing, eating, bathing, destroying things, and going to school. One day, when Billy went down to the bus stop to meet the bus to go to school, he found all of his friends huddled around in a little group, talking about the Purple Wombat.
Being a little boy, Billy was curious. So he asked them, "What's the Purple Wombat?"
"You don't know what the Purple ...