(as) common as an old shoe
Lacking in manners or refinement. I can't believe he just put his elbows on the dinner table—why, he must be as common as an old shoe!
*common as an old shoe
and *common as dirtlow class; uncouth. (*Also: as ~.) That ill-mannered girl is just as common as an old shoe. Despite Mamie's efforts to appear to be upper class, most folks considered her common as dirt.