Questions about example sentences with, and the definition and usage of "Proverbial"

A: ‘Green thumb.’ (If someone has a green thumb then they are good at keeping plants.) Example sentence: “Boy Roger, what with all those lush plants you’ve grown, you must have a green thumb!”

‘Rule of thumb.’ (A rule of thumb is a rule that is inherently understood.) Example sentence: “It’s a rule of thumb that stealing is bad! How could you not know that, Johnny?!”

‘Cat got your tongue?’ (If a cat has someone’s tongue then they can’t speak. They may trip over their own words. How awful!) Example sentence: “Huh? Why aren’t you speaking, Tom? What’s the matter, cat got your tongue?”

‘A needle in a haystack.’ (Searching for a needle in a haystack is incredibly difficult, if not impossible.) Example sentence: “Mary, looking for your lost ring in the river is just searching for a needle in a haystack!”

‘Final nail in the coffin.’ (A final nail in the coffin is the thing that makes a situation as bad as it could be.) Example sentence: “Just don’t let the government catch your tax fraud, Lucy. It’ll be the final nail in the coffin. You’ll go to jail for sure!”

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