A slur is an insulting remark. In a political campaign, it’s not unheard of for a candidate to launch a slur at her opponent, though doing so is usually frowned upon.
Slur can also be used as a verb meaning “to insult,” and it has a number of additional meanings as well. To slur one’s speech is to pronounce words in a clumsy, lazy way. In the context of music, to slur is to slide smoothly from one note to another. The word history of slur is hazy, but some scholars have suggested a relationship with a Dutch word meaning "to drag."