贱人 is a vulgar insult, literally 'cheap/lowly person'. In modern Chinese it's a strong curse word directed mostly at women, roughly equivalent to 'bitch' or 'slut' in English, though it can also be used for men in historical or literary contexts. The word carries extreme contempt and is highly offensive — learners should understand its register and avoid using it in real conversation unless quoting dialogue or discussing taboo language.