A mudslinger is a person who tries to damage someone's reputation with accusations or insults. Mudslingers are always kept busy during political campaigns.
If you spread rumors or gossip about another person, whether they are true or false, you are a mudslinger. The terms mudslinger and mudslinging began as "dirt throwing," probably from the Latin saying, "Throw plenty of dirt and some of it will be sure to stick." Some time in the late 1800s, this changed to mud-throwing, mud-gunning, and what was ultimately the term to stick, mudslinging.