Green’s Dictionary of Slang

j.b. n.1

[abbr.]

a hat made by the John B. Stetson Company.

[US]‘Dean Stiff’ Milk and Honey Route 138: The favorite adornment for the hobo head is a high-crowned and narrow-brimmed Stetson felt, which he calls a ‘J.B.’.
[US]Monteleone Criminal Sl. (rev. edn).