Something put in the way on purpose, so that it takes the knock instead of the thing behind it. The word is built that way: fend means to keep off, and -er names the thing that does it. A fender is never the valuable part — it is the panel, the pad or the low wall that stands between the valuable part and whatever is about to hit it, and it is the part you expect to find scratched.
From the word’s history
Wiktionary derives fender from the verb fend plus the agent suffix -er, so the modern word still says exactly what the thing does: the fender is the fender-off.
維基詞典把 fender 分析成動詞 fend（擋開）加上表示「做這件事的人或物」的字尾 -er。所以這個字到今天還在說它自己的工作：擋的那個。這條線索也解釋了為什麼車、船、壁爐上完全不同形狀的東西可以共用一個名字。
Wiktionary entry fender, etymology tree 'From fend + -er' (packet source_ref kaikki:3f6f69a190036ae4:412417612)
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