My arse!
Definition 1 of 1
Pro Tip 1/3Sarcasm Alert
The phrase is often used sarcastically to mock or discredit someone's claim. 
He says he's the best player on the team, my arse!
Pro Tip 2/3Very Informal
It's a very casual phrase, not appropriate for formal situations or respectful conversations. 
She's going to do all the work by herself, my arse!
Pro Tip 3/3British Usage
'My arse!' is primarily used in British English, so it's less likely to be understood outside the UK. 
You've read all these books? My arse!