Definition
An exclamation used to express disbelief or strong disagreement with what someone is saying; often implies impatience or annoyance.
"He's going to quit his job to become an astronaut?" Come off it! He's afraid of flying.
"I found a million dollars on the street yesterday." Come off it! Who would believe that?
"I'm going to run a marathon tomorrow." Come off it! You can't even finish one kilometer.