David Cassidy got curious about the words and phrases used in his working-class Irish family. He found them in other families in the neighborhood, of course. And he got curious enough to look for them, phonetically, in a Gaelic dictionary. He found them!
Mysterious phrases like, “Say Uncle!” begin to make sense when you learn that “anacal!” means “mercy!”
Cassidy wrote a book about his discoveries, called How the Irish Invented Slang.