'pickaback' originates from Middle English, specifically from the phrase 'pick a back' in which 'pick' (also spelled 'pik' in earlier forms) meant 'to lift or pick up' and 'back' meant 'the rear part of the body or the part of a person on which another may be carried.'

'pick a back' in Middle and Early Modern English fused into the single word 'pickaback'; later folk-etymological influence produced the variant 'piggyback,' which has become the more common modern form.

Initially it meant 'on or upon the back (being carried)', and this basic sense has been retained, though spelling and frequency shifted (with 'piggyback' becoming dominant).