One body riding on the back of another. A child climbs onto a grown-up's back, the grown-up holds the child's legs, and only one pair of legs does the walking. Everything this word does in English keeps that shape: a small thing sits on a bigger thing that is already moving, and gets carried along by it. What changes from sense to sense is only what the rider is (a child, a truck, a video, a bag of medicine, a person without a door card) and what the back belongs to.
From the word’s history
Wiktionary records piggyback as a corruption of pickaback, itself from pick-pack, meaning something carried like a pack. The oldest picture is therefore a load on a back, not an animal.
Wiktionary 記載 piggyback 是 pickaback 走音來的，pickaback 又來自 pick-pack，意思接近「像一包貨物那樣被背著」。所以最早的畫面是背上的一個負載，跟豬沒有關係。這條線索正好對上今天的核心畫面：有東西坐在背上，被帶著走。
Wiktionary entry for piggyback (kaikki extract, source_ref kaikki:3f6f69a190036ae4:1433370052), etymology_text supplied in this packet
登入就可以看每一支怎麼連回核心畫面。