1. To appear or become visible; make a showing; SYN. come out, turn up, surface, show up.
2. To become available.
3. When an electrical device or machine comes on, it begins to operate, usually automatically. Go off is the opposite of come on.
4. When a television or radio program comes on, it begins.
5. When you want to encourage people to do something or when you want them to do something quickly, you say "Come on!"
6. [always continuous] When you begin to feel a headache or an illness developing, you feel the headache or illness coming on.
7. When you come on a certain way, you speak or deal with people in that way.

When you think that people have done or said something improper or unreasonable, you say "Come on."