Back in the 16th century, goods were transported in packs that people carried on theirs or animals backs. The term used to describe this was “pick pack” because you would pick up a pack in order to carry it on your back.
“Pick pack” eventually became “pick-a-pack” as in pick a pack and
carry it on your back. Eventually, because an individual was picking
a pack to carry on his back, the term “pick-a-pack” became
Turns out, though, that the insertion of the “a” caused a problem
and ultimately paved the way for the original phrase “pick pack” to
become “piggyback”. Due to the pronunciation of the term as a whole,
“pick-a-pack” often sounded like “pick -i-back” which sounded like
“picky back”. This ultimately gave rise to the term “piggyback”
around this time for people carrying a pack on their back and by the
1930s, the definition further progressed to describe riding on
someone’s back and shoulders.
The pig was the only animal that sounded like “picky” and
“pickyback” became piggyback.