When you're curmudgeonly, you come across as very grumpy and unfriendly. A curmudgeonly teacher answers students' questions in a cranky and inapproachable way.
Waiting in a long, curmudgeonly line isn't much fun, since everyone around you seems irritable. Your own curmudgeonly mood might cause you to snap at your friends and complain about the weather, despite the fact that it's a beautiful day. The origin of the related noun curmudgeon is a mystery, although some experts guess at a connection to cur, or "dog."