||The phrase comes from the task of removing the tiny eggs of lice (nits) from someone’s hair and clothing, a tedious activity that required close attention and care. The word nit, which could also refer to the eggs of other insect parasites such as fleas, has been around in the language for as long as we have records (it appears in Old English around 825 as hnitu, but it has relatives in most European languages and has been traced back to an Indo-European root, so ancient has been the association of such pests with human beings).
But what seems a little odd is that the figurative sense of nit-picking, of petty criticism or fault finding, is modern. The Oxford English Dictionary records it first only in 1951, in the form nit-picker, in this helpful explanation from Collier’s: “Two long-time Pentagon stand-bys are fly-speckers and nit-pickers. The first of these nouns refers to people whose sole occupation seems to be studying papers in the hope of finding flaws in the writing, rather than making any effort to improve the thought or meaning; nit-pickers are those who quarrel with trivialities of expression and meaning, but who usually end up without making concrete or justified suggestions for improvement”. The first of these two slang terms has died out, with the second taking on much of its meaning.
Why it took so long for this expressive figurative sense to appear is a small mystery. Perhaps we had to wait for a time when the memory of the finicky nature of the task was still current, but when the need for it in industrialised countries had been greatly reduced through better hygiene and insecticides. Or perhaps the image came not from humans, but from some natural-history programme about our simian relatives, who spend much of their time literally nit-picking as part of grooming.