Used to judge behavior in social interactions, especially where manners are expected. Talking loudly on the phone in a library is considered discourteous.
Pro Tip 2/3
Cultural Sensitivity
What's discourteous in one culture may be acceptable in another; learn local customs. In some cultures, not removing shoes when entering a home is discourteous.
Pro Tip 3/3
Professionalism
In professional settings, being discourteous harms your reputation and relationships. Sending abrupt emails without greeting or sign-off can be discourteous.