Say the word slowly and you can almost hear what it names: a dry, light brushing, paper against paper, leaf against leaf. It is a small sound, so it only reaches you when everything around is quiet — which is why a rustle in a story means both that something moved and that nobody else was making a noise. From that one sound the word goes two ways. It stays a sound, as a verb and as a noun. And in American use it turns into the quiet, quick movement that makes the sound: slipping animals off a ranch by night, or putting a meal together out of nothing in twenty minutes.
The source entry says the origin is uncertain: probably Middle English rustelen from an Old English word meaning 'to make a noise', and it compares German rascheln and Dutch ritselen. Those look-alikes across languages point the same way — the word was built from the sound itself, which is why it still sounds like what it means.
來源資料明說這個字的來源不確定。可能是中古英文 rustelen，再往上接到古英文一個表示「發出聲響」的字；同時列出德文 rascheln、荷蘭文 ritselen 當旁證。這幾個語言的字長得這麼像，指向同一件事：它是照著聲音造出來的。所以今天唸起來還是像那個聲音本身。
Wiktionary etymology supplied in this packet (source_ref kaikki:3f6f69a190036ae4:1049428823:10320), which itself marks the origin uncertain
Two branches
那個窸窣的聲音
這一支就是核心畫面本身，只是換詞性看。動詞看的是「正在響」，名詞看的是「那個聲音」。兩者拼法一樣，句子裡的位置決定你讀到的是哪一個。
verb · B2
If leaves, paper, or cloth rustle, they make a soft dry sound as they move against each other; if you rustle something, you move it so that it makes this sound.
沙沙作響；發出窸窣聲
悄悄動手、快手弄到
這一支的焦點從「聽到的聲音」移到「發出聲音的那個動作」：手腳很快、不聲張、東西就到手了。要留意的是，來源把它們和聲音收在同一個字下面，卻沒有說明中間怎麼接起來，所以這個連結請當成好記的想法，不是歷史上的證據。
verb · B2
To get or put together something quickly out of whatever is available, especially a meal.
很快弄出（一頓飯、一樣東西）