'hard-liner' originates from modern English, specifically the compound phrase 'hard line' plus the agent suffix '-er', where 'hard' comes from Old English 'heard' meaning 'firm or severe' and 'line' comes from Old English 'līn' meaning 'a course of action or policy'.

'hard-liner' developed from the phrase 'hard line' (used in the early 20th century to describe a strict policy) and by the mid-20th century the agent noun 'hard-liner' emerged to denote a person who follows or advocates that strict policy.

Initially it described a strict or uncompromising policy ('hard line'); over time it came to refer specifically to people who adopt or advocate such a policy ('hard-liner').