One way to hold plop: a small stone goes into a pond. It is not loud and not sharp — one short wet knock, and then the water closes over the top of it. That single sound is the whole word. Sometimes English uses it as the name of the noise, sometimes as the falling that made the noise, sometimes as the person dropping the thing, and sometimes it is simply said out loud in a story. In every case you are hearing the same small soft landing.
From the word’s history
Wiktionary records plop as imitative of the sound, or possibly a variant of plap. So the word did not grow out of another word; someone wrote down what they heard, and every later use keeps that sound at its centre.
維基詞典記載 plop 是模仿聲音造出來的字（imitative），也可能是 plap 的變體。也就是說，它不是從別的字長出來的，而是有人把聽到的落水聲直接寫成字，後面每一種用法都還留著那個聲音。
Wiktionary entries for plop, noun/verb/interjection (kaikki extract, source_refs kaikki:3f6f69a190036ae4:1031688740 / :1031695089 / :1031701811): Imitative of the sound, or perhaps a variant of plap.
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