The Secret Origin Of "Bones"

[Dane_Whitman]

"Bones is short for "sawbones," an ancient description for a surgeon."

According to Memory Alpha.

No known onscreen reference.

Well, I vaguely recall Kirk did call McCoy "sawbones" once. I think it was in "A Piece of the Action".

--edit: I found the dialogue

MCCOY: Jim.
KIRK: Knock it off, Sawbones. I want to talk to this creep. Now listen, I'm getting tired of playing pattycake with you penny-ante operators.