A person who makes overly fine distinctions or argues about tiny, unimportant differences that most people wouldn't care about.
Compound of 'hair' (something very thin or small) and 'splitter' (one who splits), popularized in the 1600s. The metaphor works because splitting a hair would be pointlessly difficult and result in meaningless fragments.
The phrase 'hair-splitting' became so common for nitpicky arguing that it actually influenced how we describe someone's personality—if you call someone a hairsplitter, you're using a 300-year-old visual metaphor that English speakers instantly understood as absurd and pedantic.
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