Green’s Dictionary of Slang

1. a dull, boring person.

1924

G. Henderson Keys to Crookdom 404: Flat wheel. A slow fellow, stupid person.

2. one who walks with a limp.

1931		Writer’s Digest 11.41: Flat Wheel—A car wheel that has flat spots on the tread; also applied to an employee who walks lame or limps [DARE].
1958		McCulloch Woods Words 64: Flatwheel—A lame man.