Piggyback is a corruption of pickaback, which is likely a folk etymology alteration of pick pack, which perhaps is from pick, a dialectal variant of the verb pitch.
Examples
In rail transport, the practice of carrying trailers or semi-trailers in a train atop a flatcar is referred to as "piggybacking". Early drawings of the Liverpool & Manchester Railway c1830 show road coaches being piggybacked on railway flat wagons. The rail service provided for trucks which are carried on trains for part of their journey is referred to as a rolling road, or rolling highway. A related transportation method is the rail transport of semi-trailers, without road tractors, sometimes referred to as "trailer on flatcar". It is also possible to carry a railway wagon of one track gauge on a flat railway wagon of another gauge. Whole trains of one gauge can be carried on a train of flat wagons of another gauge, as was done in by the Commonwealth Railways on the Marree railway line in Australia between Telford Cut and Port Augusta in the 1950s. The loading of semi-trailers in the United States grew from 1% of freight in 1957 to 5% in 1964 and 15% in 1986.
The 1930s British Short Mayo Composite, in which a smaller, four-engine floatplane aircraft named Mercury was carried aloft on the back of a larger four-engine flying boat named Maia, enabled the Mercury to achieve a greater range than would have been possible had it taken off under its own power. The American Space Shuttle was carried on top of specially-modified Boeing 747Shuttle Carrier Aircraft when the shuttle landed at places other than Kennedy Space Center.
Space
In space transportation systems, a smaller satellite that is carried as a secondary payload on a launch is said to be "piggybacked" on the main launch. It is often the case of small satellites and cubesats, since they can not usually afford accessing space on a dedicated launch and they choose instead to take profit of the remaining payload capacity in a big satellite launch. However, this is usually at the cost of not being able to fly to their desired orbit and having to remain on a similar orbit to that of the big satellite.
Military
The metal caterpillar treads of a tankwear out quickly when travelling long distances on ordinary roads. Also, tracked vehicles seriously damage the tarmac layer of ordinary roads. It is therefore necessary to provide tank transporters, which have rubber tires, to the battlefield.
Human locomotion
A person carrying someone else on their back is most commonly seen in the modern day in the form of a parent carrying a child. It may also feature in the context of play or sport, and evidence of this dates back to Ancient Greece where games involving piggyback riding were combined with the requirement of catching or throwing a ball.
