If you're rude or inconsiderate, you're discourteous. It would be discourteous to invite only four of your classmates to your party in front of the whole class.
Burping on purpose, pushing your way to the front of a line, failing to send a thank you note for a generous gift, deliberately hurting someone's feelings, or failing to bow when you meet the Queen: these are all discourteous ways of behaving. When you're courteous, you have excellent manners and you're very thoughtful. Add the prefix dis-, meaning "lack of," and you get discourteous.