Come off it
Definition 1 of 1
Pro Tip 1/3Skepticism Expressed
Use it to show disbelief or doubt about what someone is saying. 
Come off it, there's no way you finished the project in one night.
Pro Tip 2/3Casual Tone
It's casual and best used in informal conversations among friends. 
Come off it, you're not going to wear that to the beach, are you?
Pro Tip 3/3Challenge Untruths
Employ the phrase when challenging someone who you think is not being honest. 
You've read all these books in a week? Come off it!