Put a finger on the inside of your wrist and wait. Something arrives, stops, and arrives again, always at the same distance. That regular low push is the whole of throb. It is never one event and never a flat continuous sound — it is a beat that keeps returning, strong enough to be felt and not only heard. From there the word simply changes what is doing the beating: a swollen thumb, a heart before a test, a ferry engine under the floor, or a night market so full of people that the street itself seems to have a pulse.
From the word’s history
The source traces the verb to Middle English throbben and says the origin is uncertain, possibly imitative — the word may have been made to sound like the thing it names. That fits the core: a short, blunt, repeated sound.
資料把這個動詞追到中古英語 throbben，並且說來源不確定，可能是模仿聲音造出來的字——也就是這個字本身可能就在學它所描述的聲音。這一點跟核心畫面很合：短、悶、會重複。
Wiktionary etymology supplied in this packet (kaikki:3f6f69a190036ae4:771216686:22250): 'From Middle English throbben; possibly of imitative origin. Cognate with Scots thrab.'
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