Picture the one small mark that sits exactly in the middle of the front of a body — the knot left where you were once joined to your mother. Two facts about it do all the work. First, it is dead centre, so anything with a middle can borrow the word. Second, it is unmistakably yours, and it is the one spot you have to bend your head to look at, which is why staring at it became a joke about people who only look inward. Even the orange is named from the same mark: a small round scar at one end that looks like a little navel.
From the word’s history
The source entry traces navel back through Old English nafola to a root meaning 'hub, navel' — the same word family as the centre of a wheel. A word that once covered both the middle of a body and the middle of a wheel already carries the idea of a centre.
來源資料把 navel 一路追到古英語 nafola，再上溯到一個同時表示「肚臍」和「輪轂（車輪中心）」的詞根。也就是說，這個字很早就同時指身體的中心和輪子的中心。「中央那一點」這個想法，本來就在字裡面。這是一條可以拿來記憶的線索，不必把它當成後面每個用法的直接來源。
Wiktionary, "navel"
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