The Collaborative International Dictionary of English v.0.48:

Siriasis \Si*ri"a*sis\, n. [L., fr. Gr. ???, fr. ??? the Dog
   Star, properly, scorching.] (Med.)
   (a) A sunstroke.
   (b) The act of exposing to a sun bath. [Obs.] Cf.
       Insolation.
       [1913 Webster]

WordNet (r) 3.0 (2006):

siriasis
    n 1: sudden prostration due to exposure to the sun or excessive
         heat [syn: sunstroke, insolation, thermic fever,
         siriasis]