Green’s Dictionary of Slang

(US) a hat made by the John B. Stetson Company.

1933		Buckner ‘Ranch Diction of the Texas Panhandle’ in AS VIII:1 31: john b. A hat. No hat was worthy of the name unless it was a Stetson.
1949		Monteleone Criminal Sl. (rev. edn).
1980		in DARE.