Wiktionary
born in a barn
a. 1 (context idiomatic English) Lacking a sense of etiquette; ill-mannered. 2 Of humble birth, especially when referring to Jesus Christ. 3 (context idiomatic English) Engaging in the annoying behavior of inappropriately, and usually neglectfully, leaving open a door or window.
Usage examples of "born in a barn".
He was born in a barn, and was so poor He hadn't any house to live in when He grew up, and nothing to eat sometimes, but what people gave Him, and He went round preaching to everybody, and trying to make them good, till the bad men killed Him.