Fortnight

Fortnight. It's an alteration of the either old English or Middle English Fourteen Nights Easy enough to use in a sentence. My pay will be in the bank In a fortnight.

Some people get paid fortnightly...every 2 weeks. I won't have a day off for a fortnight. There are quite a few uses for the word but it doesn't get used that much I don't think which could be why you've not heard of it until recently.

The definition given is really rather an idiomatic use of the word. "Saturday fortnight" does, indeed, mean "In two weeks from Saturday" or "not the next Saturday but the one after that". A fortnight is a period of 14 days [nights] and the word comes from a shortening of 'fourteen nights'. I am not sure why people would count in nights rather than days but we do, from time to time: excited children are told that it's "7 sleeps till Christmas", for example. You'd more usually hear the word in the phrases: "I'll see you in a fortnight" (= at some time roughly two weeks away and not next week) or "I'm going on a fortnight's holiday" (= I shall be away for two weeks) or "This magazine is published fortnightly" (= every two weeks).

Somehow we called it fortnight even through why is it called fortnight?

I seldom use it

It's two weeks! Yes, you can easily use it in a sentence.