The word "hardline" can be used to describe someone who is uncompromising, firm, or strict in their beliefs, principles, or actions. There are many synonyms for this term such as rigid, inflexible, unbending, uncompromising, staunch, and determined. These words indicate a person who is unwilling to compromise on their ideas, opinions, and actions, even in the face of opposition or criticism. Other synonyms include resolute, unyielding, unwavering, and steadfast. These words represent a strong and determined individual who is committed to their beliefs and principles, and who is willing to fight for what they believe in, no matter what the cost. The use of these words can add depth and nuance to a description of a person or situation characterized by unyielding behavior.