Lewis & Short

implăcĭdus (inpl-), a, um, adj. [2. in-placidus], ungentle, rough, savage, fierce (poet.): Genauni, implacidum genus, Hor. C. 4, 14, 10: Mars divum implacidissimus, Stat. Th. 9, 4: implacido letalis Sirius igni, Stat. S. 2, 1, 216: fores, Prop. 4 (5), 9, 14: cornu, Stat. Th. 5, 199.