Unknown, but possibly a use of pat (“light tap”) in the sense of hitting the mark.
- (idiomatic) Thoroughly practiced, rehearsed, or understood; mastered.
1913, Jack London, chapter 8, in The Valley of the Moon:
- "She's got that Indian wrestlin' down pat, an' she's built for it."
- 2008 Oct. 9, Clifford Coonan, "The Bruce Lee legend," The Independent (UK) (retrieved 4 Nov 2008):
- He has the look of Bruce Lee down pat, with the same defiant expression and spare but muscled frame.