Definition of nit

The origin of the word "nit" can be traced back to Old English, where it was spelled as "nytt" or "nyht." Its earliest meaning was "egg" specifically that of a bird such as a chicken or goose. Over time, the word's meaning evolved to refer to the eggs of other animals like fish (a "herring nit") and insects, particularly the lice that infested the hair or feathers of humans and animals. This usage became widespread during the medieval period when cleanliness and hygiene were not widely practiced. The word "nit" as a term for a parasitic insect's egg, particularly that of head lice or the sheep louse, remains in use today. Its specific meaning as a measurement for weld impurities, spark emissions, or nitrogen in the air found in an enclosed environment developed in the context of industrial manufacturing and air pollution control.