Other forms: mussed; mussing; musses
To muss is to mess up. When your grandmother reaches over to muss your hair, she tousles it — you'll have to carefully comb it again before you leave for school.
Falling in the mud might make you muss your new jeans, and a strong wind on a boat will muss everyone's hair. While the verb muss means "make untidy," it's almost always used to talk about hair, and occasionally clothing. The word muss has been around since the nineteenth century, and it was probably originally a variation on mess. A muss was once also a term meaning "a fight or disturbance."