NSW Opportunity Class
Implacable
adjective/im-PLAK-uh-bul/
Impossible to calm down, satisfy or make peace with.
In contextHer implacable determination carried her to the finish line.
Remember it
Im- (not) plus 'placate' — a person you simply cannot placate or calm.
Use it well
Use for a foe, force or feeling that never softens or gives up.