One picture carries most of this word: something shaking in small, quick, repeated movements. Not one big shock — a fine that keeps going, like a candle flame that will not sit still. Put that into the sound of a person speaking, and you have the uses you meet most often.
From the word’s history
The supplied source entry traces quaver to Middle English quaveren, a frequentative form of quaven, 'to tremble'. A frequentative names an action done again and again, which fits the modern picture: not one shake, but shaking that repeats.
這份資料裡的來源條目說 quaver 來自中古英語 quaveren，是 quaven（發抖）的反覆形式。反覆形式指的是「一做再做」的動作，正好對上今天的畫面：不是抖一下，而是一直抖。
Wiktionary entry supplied in this packet (source_ref kaikki:3f6f69a190036ae4:106985328:16772): 'From Middle English quaveren, frequentative form of quaven, cwavien (to tremble)'
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