English word fortnight comes from Old English fēowertȳne niht (fourteen nights)

Detailed word origin of fortnight

Dictionary entry	Language	Definition
fēowertȳne niht	Old English (ang)	fourteen nights
fourten night	Middle English (enm)	fourteen nights
fortnight	English (eng)	After a fortnight has passed. (chiefly, UK, Ireland, Australia, New Zealand) A period of fourteen nights; two weeks.