Fortnight fourteen days; two weeks.
Fortnight is a portmanteau word. A portmanteau is a blended word. The combination takes on the meanings of both words.
Fortnight is a combination of fourteen and night.
Change the spelling and, for some reason you end with the highly addictive game Fortnite. For that, I have no explanation.
By the way, if you are into words, you will love Word up!