Excerpts from the Steve Allen (variety) Show, 1962. Slim Gaillard

Very cool! The playing with the back of the hands and his other antics are just for show but he does seem to have an unsual way to hold his left hand. I think it's just a bad habit of turning it sideways with finger 4 sticking out like that. He might have done that to compensate for his huge hands.

Ooooh I love it!!! The music itself, his playing style (upside down? REALLY? that puts Chico Marx to shame), facial expressions, everything! And Bart you weren't kidding about his huge hands... don't think I have ever seen anything like it - he could probably span two octaves easily 🤩

I had never heard of him. Interesting life story - here's a stylised telling of it: