Picture a full bag of rice sliding off a table onto a wooden floor. The sound goes down, not up. It is short and deep and then it is simply gone — the floor seems to swallow it, and nothing rings or rattles afterwards. Every use of thud can be traced back to that one landing: an object coming down, feet coming down step after step, and a heart coming down hard inside the chest.
From the word’s history
Wiktionary traces thud to Middle English thudden 'to strike with a weapon' and Old English þyddan 'to strike, press, thrust'. So the oldest layer of the word is the striking itself, and the sound we now name with it is what that striking produces.
依照 Wiktionary 的記載，thud 來自中古英文 thudden（用武器打），再往前是古英文 þyddan（打、推、撞）。也就是說，這個字最老的一層講的是「撞上去」這個動作。我們今天用它來指那一聲悶響，可以想成從動作那一端走到聲音這一端。
Wiktionary (kaikki extract kaikki:3f6f69a190036ae4:1119864333:12686 / :1119877019:10540): Middle English thudden < Old English þyddan 'to strike, press, thrust'.
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