The word "hard-bill" has a few synonyms that carry similar meanings. One of these synonyms is "stubborn," which implies a difficulty in changing one's mind or behavior. Another synonym is "firm," which carries the connotation of a steadfast and unyielding attitude. "Inflexible" is also a synonym for "hard-bill" and implies a lack of adaptability or flexibility. "Uncompromising" is another synonym that conveys a sense of rigidity and an unwillingness to negotiate or compromise. Overall, these synonyms all convey a sense of difficulty or resistance in dealing with a person who is described as "hard-billed".