Green’s Dictionary of Slang

1. (US) a penis.

1923		Poems, Ballads and Parodies 39: Then with his nine-inch Johnson bar / He broke my maidenhead [HDAS].
c.1928		‘Boob McNutt “The Meaning of Jazz”’ [comic strip] in B. Adelman Tijuana Bibles (1997) 19: Don’t you ever speak to me again until you learn what your johnson bar is for.
c.1935		‘Mae West in “The Hip Flipper”’ [comic strip] in B. Adelman Tijuana Bibles (1997) 93: Lotta got a flash of the Johnson bar.
1992		Randolph & Legman Ozark Folksongs and Folklore I 366: Jimson or jempson-weed. This last was used at Ozark mountain midnight ‘Devil-worship’ sex orgies as inebriant, and as a presumed aphrodisiac-of-erection, the erect penis therefore being called the Johnson bar, by confusion with an actual railroading term for a large phalliform brake handle.

2. (US) a dildo.

1967		W. Crawford Gresham’s War 173: I had taken her back to my old bedroom workbench and put a Johnson Bar job on her [HDAS].
1972		B. Rodgers Queens’ Vernacular 61: dildo [...] Johnson bar.