Entry 3 senses · 2 variants Webster, 1913 Fornicate · For·ni·cate · IPA /ˈfɔɹ.nɪˌkeɪt/ a. v. i. 01 a. Vaulted like an oven or furnace; arched. 1. Vaulted like an oven or furnace; arched. 2. Arching over; overarched.(Bot.) 02 v. i. To commit fornication; to have unlawful sexual intercourse. 1. To commit fornication; to have unlawful sexual intercourse.