You can think of bonk as a noise that turned into a word. One short, dull knock: a head meeting a door, a mango meeting the path. Nothing sharp, nothing loud, nothing that lasts — it happens and it is over. Because the word sounds small and silly, almost everything it names keeps that tone: small knocks, small sounds, and joking uses of them.
From the word’s history
Wiktionary records bonk as being of imitative origin, and compares it with bang, bounce and bump. That fits what the word does: it names the noise first, and the actions come from the noise.
來源記載 bonk 是擬聲而來的字，並把它跟 bang、bounce、bump 放在一起比較。這正好對上它的用法：先有那一聲，其他意思才從那一聲長出來。
Wiktionary entry for bonk (kaikki:3f6f69a190036ae4:1006897103:29314): 'Of imitative origin. Compare English bang, bounce, bump.'
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