so-and-so
so-and-so (2) {n.}, {informal}
A person of a special kind and usually of a very bad kind. — This word is used in place of a more unacceptable word or swear word.
I wish that old so-and-so who thinks digging is easy work was right here digging now.
He called me a dirty so-and-so and I hit him in the mouth.
- Sometimes used in a joking way.
Peter met his friend John and said, "Hello there, you old so-and-so."
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Source: A Dictionary of American Idioms