“Semi” refers to the trailer, actually:
" A semi-tractor-trailer truck , also known as simply a semi-trailer truck , semi-tractor truck , semi-tractor-trailer or tractor-trailer truck , is the combination of a tractor unit and one semi-trailer or more to carry freight. A semi-trailer attaches to the tractor with a type of hitch called a fifth-wheel.
It is variously known as a transport truck, semi-truck, trailer truck, tractor truck, transfer truck, articulated truck, articulated lorry (UK), artic, single truck, semi-trailer, tractor-trailer, semi-tractor, semi, trailer, tractor, big rig, eighteen-wheeler, depending on the country and region."