fort·night (fôrt'nīt') n. A period of 14 days; two weeks.
[Middle English fourtenight, alteration of fourtene night, fourteen nights : Old English fēowertēne, fourteen; see kwetwer- in Indo-European roots + Old English niht, night; see nokwt- in Indo-European roots.]
17c. contraction of M.E. fourteniht, from O.E. feowertyne niht, lit. "fourteen nights," preserving the ancient Gmc. custom of reckoning by nights, mentioned by Tacitus in "Germania" xi.