The etymology of the word fortnight is rather bland. It’s short for “fourteen nights.” More specifically, it comes from the Old English word, “fēowertīene niht,” which has the same meaning. In the U.S., we generally use the term biweekly instead of fortnight.
In astronomy, a fortnight is a half of a synodic month (14.8 days), which is equivalent to the mean time between a full moon and a new moon.
- Wikipedia – Fortnight