The story
Blend of Texas + truckbed. From being a truck with a truckbed used in Texas oilfields. By surface analysis, Texas + bed.
Etymology from Wiktionary, CC BY-SA 4.0.
Texas bed
Today it means: A semi-truck's tractor unit, with a rear-facing near-cab winch, a truckbed between the cab and the fifth wheel, having a fifth wheel at the rear for attaching a semi-trailer. Full definition →