Implacable (adjective)

1. Unable to be pacified or soothed; unyielding.
2. Impossible to stop or prevent.

Origin:

The term 'implacable' originated in the late 16th century and is derived from the latin word 'implacabilis' meaning 'unyielding' or 'inflexible'.

Examples:

1. The implacable dictator refused to step down, even in the face of widespread popular protests.
2. The implacable enemy refused to surrender, continuing to fight even after their defeat was all but certain.
3. Despite her pleas and promises, he remained implacable, unwilling to forgive her for her past mistakes.
4. The implacable storm swept across the countryside, leaving a path of destruction in its wake.
5. The implacable disease seemed to resist all attempts at treatment, leaving doctors and researchers at a loss for how to cure it.