Oughtn't is a contraction meaning “ought not,” used to say that something shouldn't be done or isn't right. When your mom says “You oughtn't stay up so late on a school night,” she means you shouldn't do it because it's not a good idea. When a teacher mentions “We oughtn't judge people without knowing their whole story,” she's saying it's wrong to do so.

The word has an old-fashioned sound to it, like something from a classic novel. You're more likely to encounter oughtn't in older books or hear it from someone speaking formally than to use it yourself in everyday conversation. Most people today would simply say “shouldn't” instead, which means roughly the same thing but sounds more natural in modern speech.

Oughtn't carries a gentle sense of moral guidance or wisdom rather than a harsh command. It suggests that something goes against what's proper or sensible. If someone says “You oughtn't talk with your mouth full,” they're pointing out a breach of good manners, not threatening punishment.