“ The word is common in varying forms to Indo-European languages. The root is usually taken to be nak-, to perish, the word meaning the time when the light fails (cf. Gr. νέκος, Lat. nex, death, nocere, to hurt) . It was customary to reckon periods of time by nights, and we still use “fortnight” (O. Eng. feowertyne niht, fourteen nights) , but “se’n-night” (seven nights) has been displaced by “week” (q.v.) ”