You can picture a whisker as one thin, stiff hair standing straight out from the side of an animal's face. It is long enough to see easily, yet it has almost no width at all. Both halves of that picture do work in the word: the hair itself, and the fact that a hair is as thin as a thing can be. From the hair come the hairs on a man's face and the thin line drawn out of a box on a graph; from the thinness comes a gap so small that only a hair fits into it.
From the word’s history
The noun comes from the verb whisk plus -er, that is, 'something that whisks' — something that sweeps quickly across. The stiff hairs beside an animal's mouth, which brush through the air as it moves, were named from that quick sweeping motion.
這個名詞可以追到動詞 whisk（快速掃過）加上 -er，字面上是「會掃動的東西」。動物走動時，嘴邊那幾根硬毛正好在空氣裡掃來掃去，名字就這樣定了下來。
Wiktionary entry for whisker, supplied in the evidence packet (source_ref kaikki:3f6f69a190036ae4:747557082:47567): 'Inherited from Middle English wisker, whisk (verb) + -er.'
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