Green’s Dictionary of Slang

1. a fast driver [the heaviness of a foot on the accelerator].

1942		B.H. Porter ‘Truck Driver Lingo’ in AS XVII:2 Pt 1 104/1: Lead Foot stepping on the gas.
1948		Hot Rod Mag. Nov. 27/1: Leadfoot – A driver who uses a heavy throttle, meaning that he is a fast driver.
c.1950		in D. McKay Wild Wheels 60: ‘Leadfoot’ Murphy...his foot ‘heavy’ on the accelerator [HDAS].
1970		J. Wambaugh New Centurions 49: Sometimes I’m a leadfoot. I can’t help stomping down [on the accelerator] on a four-five-nine call.
1986		Daily Beacon (Univ. Tenn.) 20 Nov. 5: Drivers here also are speed demons and leadfoots for the most part [HDAS].

2. a clumsy person [the heaviness that slows one down].

1955–57		Felder Collegiate Sl. 3: Leadfoot — a male dancing partner [HDAS].
1958		McCulloch Woods Words 105: Leadfoot — A slow or clumsy logger.