灭绝人性 is a four-character idiom (成语) meaning 'completely devoid of human decency' or 'inhumanly cruel'. It describes actions or people that are so brutal or barbaric they seem to have lost all human compassion. You can use it as a predicate or attributive modifier — for example, 灭绝人性的行为 ('inhuman behavior'). It's a strong, condemnatory term, typically used in formal or literary contexts.