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It's like a rule you use when you're too lazy to think, and you just say it because you heard it on the internet and now you're a fake expert.
I used the fact of thumb to figure out my math homework. I didn't even try.
She said, 'I'm using the fact of thumb,' and I asked, 'What does that even mean?'
He failed the test but still said, 'I used the fact of thumb.'