Make trivial distinctions, quibble, as in Let's not split hairs about whose turn it is; I'll close up today and you do it tomorrow. This metaphoric idiom transfers dividing so fine an object as a single hair to other petty divisions. [Second half of 1600s]

To argue about an inconsequential and trivial aspect of an issue: “When you are accused of being forty-five minutes late for an appointment, you are splitting hairs to say that you were really only forty minutes late.”