Archaic. a week.
It had taken them only a sennight to travel from Sentarshadeen … into the heart of the lost Lands to face the power of Shadow Mountain.
Mercedes Lackey and James Mallory,To Light a Candle, 2004
She that I spake of, our great captain’s captain, / Left in the conduct of the boldIago, / Whose footing here anticipates our thoughts / A sennight ‘s speed.
William Shakespeare, Othello, 1622
The archaic English noun sennight means literally “seven nights,” i.e. a week.
The Old English form was seofan nihta.
Middle English had many forms,including soveniht, sevenight, seven nyght, sennyght.