Other forms: mucked up; mucking up
When you muck up, you get something dirty. Your grandmother might instruct you to take off your muddy boots outside, so you don't muck up her rugs.
You can muck up the floor with your dirty sneakers, and you can muck up a brownie recipe by ruining it (if you add salt instead of sugar, for example, you will definitely muck it up). You could also say you "mess up," "bungle," or "goof up." Muck up comes from the figurative meaning of the verb muck, "make a mess of" as well as the literal meaning, "make dirty."