As frank panda says, it means "fourteen nights". It derives from the Old English "fēowertyne niht" (fourteen nights). It was shortened over time by common usage. It should be noted that almost nobody in the United States will know what you mean if you talk about a fortnight. On the other hand, anybody who has learned English in the British Isles, or any of the Commonwealth countries, will understand it and use it.

origin :"fourteen nights"

oh~~ thank you ~