Definition of "fortnite"

We are all together in the same house, and all parties have agreed to the match, and in a fortnite the surrymony will be performed.

1771, [Tobias Smollett], The Expedition of Humphry Clinker […], volumes (please specify |volume=I to III), London: […] W. Johnston, […]; and B. Collins, […]

Voted, by this company, to continue their meetings once a fortnite on mondays at two of ye clock in ye afternoon.

1772 October 2, Oscar Jewell Harvey, editor, The Town Book of Wilkes Barre, quoted in A History of Wilkes-Barré, Luzerne County, Pennsylvania, published 1909, page 750

Within a fortnite several others followed suit.

1953 August, The Rotarian, volume 83, number 2, page 51

I think the blocks were saturated and held at 50° C and the water was changed every day for a fortnite or something like that.

1972, Report of Proceedings, volume 68, American Wood-Preservers' Association, page 28