Fortnight is a contraction of the Old English féowertyneniht, literally meaning fourteen nights. From the manuscript Laws of Ina, dated to sometime before 1000:
Oþ ðæt feowertyne niht ofer Eastron.
(Until the fourteen nights of Easter.)
The form fortnight developed in the 13th century. We can see this in different versions of a 13th century poem. The first, from c.1205:
Nou his folle feowertene niht þat he hire haueþ i-holde forþ riht.
(Now his full fourteen nights that he has held her forth right.)
And the second, from c.1275, substitutes fourteniht for feowertene niht.
In addition to fortnight, there used to be the term sennight, from the Old English seofon nihta (seven nights). This remained in use through the 19th century, but eventually became obsolete, leaving us with just fortnight.
(Source: Oxford English Dictionary, 2nd Edition)
Copyright 1997-2015, by David Wilton