I had never heard this before, either. According to Merriam-Webster Dictionary, using “pat” as an adjective can mean 1a: Exactly suited to the purpose or occasion. 1b: Suspiciously appropriate. 2: learned, mastered, or memorized exactly. 3: FIRM, UNYIELDING. 4: reduced to a simple or mechanical form.
Thanks! I was looking for that, too.