im·plac·a·ble 
(ĭm-plăkə-bəl, -plākə-)
(ĭm-plăkə-bəl, -plākə-)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·bili·ty n.
im·placa·bly adv.