'nit' (the sense 'louse egg') originates from Old English, specifically words such as 'hnitu' or 'nitu', where a Proto-Germanic root (reconstructed as '*hnit-' or similar) referred to a 'louse egg'. The modern technical sense 'nit' (unit of luminance) is a 20th-century coinage formed in English from roots meaning 'shine' (influenced by Latin 'nitēre' 'to shine') to denote brightness.

'nit' changed from Old English forms like 'hnitu' to Middle English 'nit' and eventually to the modern English 'nit' for the louse-egg sense. Separately, in the 20th century the word was adopted as an informal name for a luminance unit and became established in technical contexts.

Initially, 'nit' meant 'a louse's egg' and that meaning has persisted into modern English. Over time, a new technical meaning ('unit of luminance') was added in the 20th century, so the word now has both the traditional biological sense and a modern technical sense related to brightness.