"period or space of seven days and nights, a week" (archaic), late Old English sefennnahht (see sennight).
Entries linking to sevennight
sennight n.
"period or space of seven days and nights, a week" (archaic), a contraction by late 14c. of late Old English sefennnahht (Orm), itself from Old English seofon nihta literally "seven nights;" see seven + night. Also compare fortnight.