Picture a main thing, and then a smaller thing hanging off the side of it. The main thing would still be itself without the smaller one; the smaller one would be nothing much on its own. That is the whole of appendage, and the word wears its own history on its face: it is append, to add on, turned into a noun. Everything it names fits the picture. A room added to a house. A bike lane added to a road. A person who only counts because of the group they are next to. And bodies, where the parts that stick out from the trunk are called appendages for exactly the same reason.
From the word’s history
From append plus -age; the packet's tree runs append back through Old French apendre to Latin appendo, itself ad- plus pendo.
來源資料給的是一條字源鏈：appendage 來自 append，往上經過古法語 apendre，再上去是拉丁文 appendō（由 ad- 加 pendō 組成），字尾 -age 也是經古法語從拉丁文來的。所以這個字從頭到尾都在講「加上去的東西」，這正是兩個意思共用的那張圖。
packet evidence: Wiktionary etymology tree for appendage (source_ref kaikki:3f6f69a190036ae4:692709508:26382) — Latin ad- + pendo > appendo > Old French apendre > Middle English appenden > English append; -age < Latin -aticum.
登入就可以看每一支怎麼連回核心畫面。