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!