a soft, wet, and sticky and/or slimy mixture of earth/soil and water;
e.g. The rain turned the dirt path into mud and made it impossible to walk on.
(noun)
figuratively, claims or accusations that could damage someone's reputation, or broadly, information or remarks that are harmful or abusive in nature (often used in the phrase "sling mud");
e.g. I am worried that they will sling mud at me, which will ruin my chances of being elected.