im·plac·a·ble (ĭm-plāk'ə-bəl, -plā'kə-) adj. Impossible to placate or appease: implacable foes; implacable suspicion.
[Middle English, from Old French, from Latin implācābilis : in-, not; see in-1 + plācābilis, placable; see placable.] im·plac'a·bil'i·ty, im·plac'a·ble·ness n., im·plac'a·bly adv.