split hairs

1. 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.”

Idioms and Phrases

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]