WordWeb Online

1. Expression of encouragement
   "c'mon! You can still win";
   - come on
    
2. Expression of disbelief
   "c'mon! You must be joking";
   - come off it [informal], come on, my foot [informal]
    
3. Exclamation to tell someone to be quicker
   - chop-chop [informal], buck up [informal], hurry up, come on, get a move on [informal]

Encyclopedia: C'mon, Have A Go!