ETYM Latin implacabilis; pref. im- not + placabilis: cf. French implacable. Related to Placable.
Impossible to placate; not to be appeased; irreconcilable.
ETYM Latin intractabilis: cf. French intraitable, formerly also intractable. Related to In- not, and Tractable.
Not tractable; difficult to manage or mold.
Having or showing no mercy; SYN. unmerciful.
Not remitting; incessant; continued.