'Bù guāi' literally means 'not obedient' — 'bù' is the negation particle meaning 'not', and 'guāi' means 'well-behaved', 'obedient', or 'compliant'. Together, they form an adjective describing someone (often a child or pet) who disobeys rules, acts unruly, or refuses to follow instructions. It carries a mild, affectionate or scolding tone — not harsh like 'disobedient' in formal English, but closer to 'naughty' as used by parents or pet owners.

The term is commonly used in everyday speech when addressing young children ('Don’t be naughty!') or describing pets ('The cat is being naughty again'). It’s rarely used for adults unless playfully or ironically. Though 'guāi' alone can be a compliment ('You’re so well-behaved!'), adding 'bù' instantly shifts it to a gentle reprimand. The word appears frequently in parenting contexts, children’s stories, and pet training discussions — especially with dogs and cats that misbehave.