Johnny Tinplate, n. — Green’s Dictionary of Slang

Johnny Tinplate n.

also tin plate

[a mocking allusion to his badge]

(US) a rural sheriff.

1917		in Editor 24 Feb. 153: Get your orders – To be met by a Johnny Tinplate, or constable, and be told to leave town [HDAS].
1921		P. & T. Casey Gay-cat 40: Every Johnny Tinplate has made us beat it out o’ them towns.
1940		C. Rawson Headless Lady (1987) 34: The fixer walks into Johnny Tin Plate’s office and says, ‘No grieft at all this year, Chief’.
1956		M. Murray Circus from Rome to Ringling n.p.: Johnny Tin Plate, Small town marshal or constable.
1981		D.B. Wilmeth Lang. of Amer. Popular Entertainment 146: Johnny Tin Plate or tin plate: Circus name for a small-town constable or marshall.