Explanations/etymology also appreciated!
For Joe Shmoe, it means a very average or below average person. It’s a derivation of the practice of using “shm-” to dismiss something (eg “Practice shmactice. We’re already perfect”).
And “John Smith” is meant to be the most average name or person imaginable, so they have the “most common” (citation needed) first and last name as well.
In Poland typically, both officially and informally, one says N. N. (Latin nomen nescio, identity unknown).
What the hell, dude? No one says that. Aren’t you thinking of a John Doe?
Polish “John Smith” would be “Jan Kowalski”. It is used to refer to an average citizen and also literally means “John Smith”.
Fair point, I assumed OP meant people of indeterminate identity. Jan Kowalski is indeed the goto placeholder name.