Source : Webster's Revised Unabridged Dictionary (1913)

Fornicate \For"ni*cate\, Fornicated \For"ni*ca`ted\, a. [L.
   fornicatus, fr. fornix, -icis, an arch, vault.]
   1. Vaulted like an oven or furnace; arched.

   2. (Bot.) Arching over; overarched. --Gray.

Fornicate \For"ni*cate\, v. i. [L. fornicatus, p. p. of
   fornicari to fornicate, fr. fornix, -icis, a vault, a brothel
   in an underground vault.]
   To commit fornication; to have unlawful sexual intercourse.

Source : WordNet®

fornicate
     v : have sex without being married