This article is about physical nitpicking, for the sport, see Nitpicking (sport).
Nitpicking is the act of removing nits (the eggs of lice, though in this case specifically head lice) from one's hair. The original way of treating a person for head lice was to remove each nit one at a time from the hair. This was a slow and meticulous process, and new methods that take less time have been developed in more recent times.
In modern times, the word isn't widely used in the sense of removing head lice anymore, but rather refers to unjustified criticism that is very meticulous.
Nitpicking is the act of removing nits (the eggs of lice, generally head lice) from the host's hair.
As the nits are cemented to individual hairs with louse saliva, they cannot be removed with lice combs and, before modern chemical methods were invented, the only options were to shave all the host's hair or to pick them free one by one.
As nitpicking inherently requires fastidious, meticulous attention to detail, the term has become appropriated to describe the practice of meticulously searching for minor, even trivial errors in detail (often referred to as "nits" as well), and then criticising them.