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

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "fornicate":
   ball, be intimate, be promiscuous, chase women, cohabit,
   come together, commit adultery, copulate, couple, cover, debauch,
   diddle, dissipate, frig, grovel, have sex, have sexual relations,
   lay, lie with, make it with, make love, make out, mate, mount,
   philander, rake, screw, serve, service, sleep around, sleep with,
   swing, wallow, wanton, whore, womanize