IPA: /ˈhɛrˌsplɪtə/
KK: /ˈhɛrˌsplɪtər/
A person who makes very small or overly detailed distinctions, often in arguments or reasoning.
He is such a hairsplitter that he can argue about the smallest details of a contract.
Hairsplitter → It is formed from "hair" (from Old English *hæġer*, meaning the fine strands growing from the skin) and "splitter" (from "split" which comes from Old English *splittan*, meaning to divide). The word refers to someone who divides or makes fine distinctions, often over trivial matters, much like splitting hairs.
Think of someone who is overly concerned with tiny details, as if they are 'dividing' ('split') the 'fine strands' ('hair') of an argument.