scat signing 

A series of gestures that look as if they should be meaningful but are actually complete nonsense.

“Bottom dances pretty cool, but what’s he doing with his hands?”
“He can’t sing, so must resort to scat signing.”

scat signing by Bottom Ford, Esq December 17, 2010

scat singing 

Scat singing is vocalizing either wordlessly or with nonsense words and syllables as employed by jazz singers who create the equivalent of an instrumental solo using only the voice. Thus it is a type of voice instrumental.

Scat also refers to the sexual fetish known as coprophilia, but scat singing is much cooler.

Definition from Wikipedia. It's worded well.

Famous scat man: John Larkin, otherwise known as Scatman John - his songs are good examples of scat singing.

Others: Dizzy Gillespie, Cab Calloway, etc.

scat singing by emilly December 2, 2005