sennight

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