genethliac /d͡ʒəˈnɛθ.li.æk/noun 2adjective 1forms 1 noun2 •A birthday poem. •A person who casts horoscopes.The follie of our genethliaks, or nativiti-casters. — 1584 adjective1 •Of or pertaining to the casting of horoscopes at a person's birth formsgenethliacs