Fact of thumb - It's the thing you say when you're too dumb to remember the

It's the thing you say when you're too dumb to remember the real rule, but you still pretend you're smart because of the internet.

He said, 'I used the fact of thumb, so I'm smart.'

She asked, 'What's the rule?' I said, 'I used the fact of thumb.'

My friend failed the quiz but said, 'I used the fact of thumb.'