Nitpick

To nitpick is an American expression that means to indulge in petty criticism or fault finding and dates from the early 1950s. This figurative meaning derives from the rather distasteful allusion to literally picking nits or lice from one’s hair. This used to be a regular procedure in earlier times but it remains a bit of a mystery why this figurative meaning should only appear in relatively modern times like the 1950s.