The story
The term was popularized in the late 1970s when manufacturers like General Motors (with the GMC C15 Heavy Half and Chevy C10 Big 10) offered special packages.
Etymology from Wiktionary, CC BY-SA 4.0.
heavy half
Today it means: A light-duty pickup truck (a "half-ton") that has been upgraded with heavier-duty components, such as a stronger suspension and larger axles, to increase its payload and towing capacity. Full definition →