Old English fēowertīene niht 'fourteen nights'.

• night from Old English:

  Although an Old English word, night comes ultimately from the same root as Latin nox, the source of equinox (Late Middle English) and nocturnal (Late Middle English). Fortnight (Old English) is an Old English contraction of ‘fourteen nights’, and reflects an ancient Germanic custom of reckoning time by nights rather than days. The original night of the long knives was the legendary massacre of the Britons by the Saxon leader Hengist in 472. According to the 12th-century Welsh chronicler Geoffrey of Monmouth, the Saxons attended a meeting armed with long knives, and when a prearranged signal was given each Saxon drew his weapon and killed the Briton seated next to him. The phrase is now more commonly associated with the brutal suppression of the Brownshirts (a Nazi militia replaced by the SS) on Hitler's orders in 1934. It is also used of any decisive or ruthless sacking, in particular the occasion in 1962 when British Prime Minister Harold Macmillan dismissed a third of his cabinet at the same time. Nightmares are nothing to do with horses. In the Middle Ages a nightmare (Middle English) was thought of as an evil female spirit or monster that lay on sleeping people and suffocated them: the -mare part comes from Old English and meant ‘suffocating evil spirit’.