Fact of thumb - It’s the excuse you use when you’re wrong, but you still act

It’s the excuse you use when you’re wrong, but you still act like you’re right because the internet told you so.

He got the answer wrong but still said, 'I used the fact of thumb.'

She texted me, 'I did the fact of thumb, and I’m right.'

My teacher said, 'You used the fact of thumb, but that doesn’t mean it’s true.'