Entry 3 senses · 2 variants Webster, 1913 Genethliac · Ge·neth·li·ac · IPA /d͡ʒəˈnɛθ.li.æk/ a. n. 01 a. Pertaining to nativities; calculated by astrologers; showing position of stars at one's birth. 1. Pertaining to nativities; calculated by astrologers; showing position of stars at one's birth. 02 n. A birthday poem. 1. A birthday poem. 2. One skilled in genethliacs.