'hair-splitter' originates from Modern English, a compound of the noun 'hair' (from Old English 'hær', meaning 'a strand of hair') and the agent-forming element of the verb 'split' (from Old English 'splītan'), where 'split' meant 'to divide or separate.'

'hair-splitter' developed from the idiomatic phrase 'to split hairs' (recorded in use from the 17th–18th century) meaning to make overly fine distinctions; the verbal/idiomatic use of 'split hairs' led to the noun form 'hair-splitter' to describe a person who does this.

Initially the phrase could be understood more literally as dividing a hair; over time it evolved metaphorically to mean 'to argue or make distinctions over trivial matters,' and 'hair-splitter' came to be used pejoratively for someone who does this.