On the farm, sometimes, weeds will sprout out of cow patties. They are called "shitweeds". The term is also used for a person who is miserably disagreeable, an asshole or a jerk, but has a greater depth of meaning. It represents someone who is bad and worthless (like a weed) who was spawned by something equally worthless (like shit). A person who is a "shitweed" is similar to white trash, a low and base person raised by low and base people.
Man #1: "Did you meet Joan's new boyfriend?"
Man #2: "Yeah, he's a piece of work, isn't he?"
Man #1: "I know him and his family, they are all garbage. The guy is a real shitweed!"