nit
keep nit keep watch or act as a guard. Australian
☞ Nit here is possibly an alteration of nix, a warning signal by schoolchildren that a teacher is approaching.
pick nits look for and criticize small or insignificant faults or errors.
☞ The image here is of the painstaking removal of tiny parasitic nits (lice or lice eggs) from someone's hair. The phrase originated in the mid 20th century, chiefly in North American usage.