"Semi truck" is not half a truck, but a truck designed to carry one half the weight of the cargo it is hauling. A semi trailer is one designed to have half of its load (by weight) carried by the tow vehicle. A standard trailer gets difficult and possibly dangerous to tow if the weight carried by the tow vehicle (hitch weight) strays too far outside the 8%-12% range.

And just to add to the confusion, Dodge popularized something called the "hemi engine"--an engine with a "hemi head", not half an engine. And "hemi head" refers not to "1/2 an engine head" but to the approximately hemispherical (1/2 sphere) shape of the combustion chambers cast/machined into the engine head.