fortnight: [13] The ancient Germanic peoples recorded the passing of time in units of ‘nights’ rather than, as we do, in units of ‘days’: hence a period of two weeks was in Old English fēowertīene niht, or ‘fourteen nights’. By early Middle English times this was starting to be contracted to the single word fortnight. (The parallel sennight ‘week’ [15] – literally ‘seven nights’ – survived dialectally into the 20th century.) => fourteen[fortnight etymology, fortnight origin, 英语词源]