The egg of a louse or other parasitic insect. Also means a minor criticism or trivial fault-finding.
From Old English 'hnitu', related to Germanic words for louse eggs. The meaning of 'minor criticism' developed from the phrase 'nitpicking' - the meticulous process of removing louse eggs, suggesting excessive attention to tiny details.
The phrase 'nitpicking' transformed a literal hygiene practice into a metaphor for excessive criticism. Medieval people actually did pick nits from hair as routine grooming - imagine how tedious that was, and you understand why we use it to describe overly detailed criticism.
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