'piggyback' has nothing to do with pigs. In the 20th century, we settled on the 'piggyback' spelling, but the term has existed in myriad forms over the years - 'a pick-apack', 'pick-a-back', 'pig-aback' and so on. The earliest forms of 'piggyback' were the medieval 'pick back' and 'pick pack'. These are cited within a year of each other in the 1560s, ….
https://www.phrases.org.uk/meanings/piggyback.html
