In his splendid book, The Joys of Yiddish, Leo Rosten gives two definitions of the Yiddish term Chaim Yankel:
1. A nonentity, a nobody, any “poor Joe.”
2. A colloquial, somewhat condescending way of addressing a Jew whose name you do not know—just as “Joe” or “Mac” is sometimes used in English.
He then illustrates it with the following joke:
A Chaim Yankel, visiting a cemetery, beheld a magnificent marble mausoleum, on the portal of which was incised: ROTHSCHILD.
“Ai-ai-ai!” exclaimed the Chaim Yankel. “Now that’s what I call living!”
Chaim Yankel is pronounced as one word, Khym-yonk-’l (not “chime,” but with a throat-clearing kh sound); rhymes with “dime jonquil.”
This is on pages 66-67 of my paperback edition.