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.
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
Four branches
真的趴在背上
這一組就是核心畫面本身，只是分成三種說法：名詞講「那一趟」，形容詞和副詞講「那種姿勢」，動詞講「揹」這個動作。
noun · B1
A ride on someone's back or shoulders. The person carrying you holds your legs, and you hold on around their neck or shoulders. You usually say give someone a piggyback.
揹在背上（的一程）
貨物坐上別人的背
背上的不再是人，而是一件東西。大的那個已經在動或已經在運作，小的那個掛上去，自己完全不出力。
verb · C1
To move a truck or its trailer by loading it onto a flat railway wagon, so the road vehicle travels on top of the train. The same word goes in front of nouns in this area too: piggyback freight, a piggyback service.
把卡車載在火車上運送
靠別人的力量往前
背變成抽象的了：別人的名氣、熱度或成果已經在往前走，你把自己的東西掛上去，被一起帶著跑。
verb · B2
To use something bigger, more popular or more successful as a way of helping your own product, plan or idea do well, by attaching yours to it.
搭順風車；依附壯大
蹭別人的通道與權限
這一組更進一步：你搭的是別人已經接好的線路或已經取得的許可，而且通常沒有得到同意。「沒問過」幾乎就寫在字裡面。
verb · B2
To get onto the internet through someone else's wi-fi without their knowledge or permission, so you are using a connection you do not pay for.
偷連別人的 Wi-Fi