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.
译文:
吹毛求疵、咬文嚼字
在头发和衣服里捉虱子卵是很枯燥乏味的事情，但又不得不做，捉虱子卵（nit-picking）这种用法就出自对这种行为的描写。虱子卵（nit）一词也指其他寄生虫的卵，如跳蚤卵。该词从有记载以来就出现在我们的语言里（它最早出现大约是在公元825年左右，在古英语中被拼写成hnitu，在大多数欧洲语言里都可以找到它，而且可以追溯到它的印欧语系词根。因此远在古代，人们就把这种害虫和人类联系在一起了） 。
但在现代用法中，它的意思转化为咬文嚼字、吹毛求疵、挑剔（nit-picking）似乎让人觉得有点奇怪。只是在1951年，《牛津英语词典》才第一次登录了该词，科利尔杂志对该词的解释对我们很有帮助： “五角大楼两个长期的角色就是无事生非和吹毛求疵。无事生非的人（fly-specker）指那些专门在别人的文章里想找到文字漏洞，而不是设法对意思和内容进行修改的人；吹毛求疵的人（nit-pickers）指那些为文字表达和字面意思的小事与人争吵不休，却又提不出具体或合适的改进建议的人” 。俚语fly-specker的大多数意思已由nit-picker取代了，因此也不再流行了。
为什么这个词花了这么久的时间才完成了它富有表达的转义，至今仍不太清楚。也许在过去相当长的一段时间里，吹毛求疵的本性一直在左右着人们，而由于工业化国家卫生条件的改善以及杀虫剂的广泛应用，没有人再生虱子了，其最初的意思才进化过来。或者，捉虱子卵这种行为并非来自我们人类，而是来自人类的近亲类人猿的某些自然现象，作为清洁的一部分，它们确实花费了大量时间捉虱子卵。