If someone sullies something, they make it dirty. I felt loath to sully the gleaming brass knocker by handling it. to spoil or reduce the value of something that was perfect (Probably from soiller; SOIL)
If something is sullied by something else, it is damaged so that it is no longer pure or of such high value. The City's reputation has been sullied by scandals like those at Lloyd's She claimed they were sullying her good name
make dirty or spotty, as by exposure to air; also used metaphorically; "The silver was tarnished by the long exposure to the air"; "Her reputation was sullied after the affair with a married man"
charge falsely or with malicious intent; attack the good name and reputation of someone; "The journalists have defamed me!" "The article in the paper sullied my reputation"