You can think of unsteady as one picture: something that is not being held firm. Put a ladder on soft ground and it rocks whenever you move; it has not fallen, and you would not stand on it either. Everything the word does comes from that rocking. Sometimes the rocking is in a body, sometimes in a voice, sometimes in a peace deal or a month's income — but in every case the thing is still up, still moving, and not to be counted on.
From the word’s history
The supplied Wiktionary entry gives the word as un- + steady, appearing in English around 1530, alongside steady itself. So the meaning is built in the open: steady is being held firm, and un- takes that away. Looking for the matching steady phrase is still the quickest way to place any use of the word.
來源資料把這個字拆成 un- 加 steady，並說它和 steady 一樣大約在 1530 年代出現在英文裡。所以它的意思是攤開來擺著的：steady 是「被穩住」，un- 把這件事拿掉。想判斷某個用法屬於哪一格，最快的方法還是把它換成對應的 steady 說法看看。
Wiktionary (kaikki extract supplied in this packet; source_ref kaikki:3f6f69a190036ae4:1048448929:13896): “From un- + steady. Like steady, the word first appeared in English around 1530.”
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