sennight

Etymology

Came from Middle English senight, senyght, sinight (“sevennight, a week”) [and other forms], itself a shortened form of seven-night, sevenight, sevenyght [and other forms], from Late Old English seofeniht (compare seofonnihte, “seven days old; seven days long”, an adjective), formed from seofon (“seven”), ultimately from Proto-Indo-European *septḿ̥ (“seven”), and niht (“night; day, when computing spans of time”), ultimately from Proto-Indo-European *nókʷts (“night”). It is a doublet of sevennight.