The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. (Bot.) Arching over; overarched. --Gray.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fornicate
    v 1: have sex without being married