The Collaborative International Dictionary of English v.0.48:

Quintan \Quin"tan\, a. [L. quintanus, fr. quintus fifth, quinque
   five. See Five.]
   Occurring as the fifth, after four others also, occurring
   every fifth day, reckoning inclusively; as, a quintan fever.
   -- n. (Med.) An intermittent fever which returns every fifth
   day, reckoning inclusively, or in which the intermission
   lasts three days.
   [1913 Webster]