The origin of the idiom "be born yesterday" is not definitively known, but it likely stems from the literal sense of being recently born and therefore lacking knowledge or experience. The phrase itself has been in use since at least the early 20th century, appearing in written works and popular culture. It is a figurative expression that draws upon the notion of a newborn infant having no previous exposure to the world and being easily fooled or misled due to their lack of understanding. Over time, the idiom has become a common way to express skepticism or doubt about someone's naivety or gullibility.