Marty replied: At least one other dictionary has it as "British and Australian". I know from previous discussions on the board that our Kiwi mates also use it. Try using the Search function for the word 'fortnight' (ignoring the thread "Challenge of the Fortnight"). You might like to also search for the much rarer 'sennight' (= seven days, a week) which old (cool) hand tsuwm cast before a stunned readership way back when.
Marty has it dead right. "Se'ennight' was a term used in a diary by an Otago gold miner in the 1860s, Alphonse Barrington. He used it to describe how long he was stuck in a tent under snow in a particularly inhospitable (and goldless) piece of Godzone. Apart from that I've only seen it used by the authors of historical novels like Georgette Heyer and Susan Howarth. It was (apparently) in use in Britain by the "upper crust" during the early part of the 19th Century (NO, I'M NOT GOING DOWN THAT ROUTE AGAIN!).
Fortnight is just a contraction of "fourteen nights". Your dictionary is probably right about its roots.
