The Collaborative International Dictionary of English v.0.48:
Agnus castus \Ag"nus cas"tus\ [Gr. ? a willowlike tree, used at a religious festival; confused with ? holy, chaste.] (Bot.) A species of Vitex (Vitex agnus castus); the chaste tree. --Loudon. [1913 Webster] And wreaths of agnus castus others bore. --Dryden. [1913 Webster]