A smart aleck is someone who's a wisecracking know-it-all. Greet a smart aleck by asking, "What's up?" and they're sure to answer, "The sky."
Smart alecks can be funny, but their humor is usually tinged with an obnoxious arrogance. You could also call the smart alecks in your life "wise guys." If your friend calls you on the phone and asks, "What are you doing?" and you can't help answering, "Talking to you on the phone," you're a smart aleck. Some experts think this term was coined after a nineteenth century con man named Aleck Hoag.