fortnight

From Middle English fourtenyght, fourtene nyght, from Old English fēowertīene niht, literally "fourteen nights." It's a compound of fourteen and night.

The phrase "fourteen nights" fused over time into a single word, keeping its original meaning of a fourteen-day span intact.

It's built the same way as sennight, an old word for a week that literally meant "seven nights" — both reflect a time when the Anglo-Saxons counted stretches of time by nights rather than days.