Shan't is a contraction of “shall not,” used mainly in British English. If you tell your little brother “You shan't have any cookies before dinner,” you're saying he will not be allowed to have them. When a character in an old-fashioned story declares “I shan't go!” they're refusing firmly and clearly.

You'll encounter shan't mostly in British books, movies, and speech, where it sounds perfectly normal. In American English, people usually say “won't” instead. A British student might say “I shan't be late,” while an American student would more likely say “I won't be late.”

The word has a somewhat formal or old-fashioned feeling to it. You might hear it in historical movies or classic British literature more often than in everyday conversation. Like other contractions (can't, won't, don't), shan't combines two words into one to make speech flow more smoothly.