Crossword Dictionary
muss
muss, tussle - v
make messy or untidy; "the child mussed up my hair"
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."
etymology
"to make untidy, put in a state of disorder," 1837, American English, probably a variant of messin its sense of "to disorder." It was attested earlier (1830) as a noun meaning "disturbance, state of confusion." Related: Mussed; mussing.