noun. The painstaking process of removing nits (lice eggs) from someone's hair.

noun. A process of finding or pointing out tiny details or errors, particularly if the pointed-out details seem insignificant or irrelevant to all but the finder.

"I can't remember how old I was when I figured out that the term nitpicking must have come from picking lice nits."

nitpick  nitpicker  nitpicky  

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nitpick