Unyielding; obstinate.

And even as they did not like to retain God in their knowledge, God gave them over to a reprobate mind, to do those things which are not convenient; being filled with all unrighteousness, fornication, wickedness, covetousness, maliciousness; full of envy, murder, debate, deceit, malignity; whisperers, backbiters, haters of God, despiteful, proud, boasters, inventors of evil things, disobedient to parents, Without understanding, covenantbreakers, without natural affection, IMPLACABLE, unmerciful. ( Romans 1:28-31 )