to behave badly towards someone in a way that shows that you do not respect them.
I don't know why she stays with him. He treats her like dirt.
treat (one) like dirt
To treat one very poorly or disrespectfully; to treat one as having no value. I'm not surprised that so many of your employees are quitting—you treat them like dirt!
treat like dirt
Behave badly or show contempt toward, as in Her boss treats all the secretaries like dirt. This idiom uses dirt in the sense of "something worthless," a usage dating from the mid-1300s.