It is considered unlucky to have thirteen people seated at a table because it's believed that the first person to leave the table will die.

This idiom has its origins in The Last Supper. Judas Iscariot was the thirteenth man seated at the table with Jesus and the twelve Apostles and after betraying Jesus to the Roman authorities, he left the table and hanged himself.

Example of use:
We can't possibly have thirteen at table tomorrow. We'll have to invite some more people for dinner.

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A closed book is something or somebody that is a mystery; something or somebody that you know nothing about.

Example of use:
Our new neighbour is a closed book. Nobody knows who he is, or where he's from.

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A cowboy outfit is a business that operates without moral principles; one that cheats and deceives its customers by providing a very bad service and taking money for it.

Example of use:
Don't book a flight with that airline; they're a cowboy outfit. The plane was filthy, the food was horrible, the staff were rude, and they lost my luggage.

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