Other forms: mussing
To muss is to make something untidy by ruffling it. Muss his hair, mussed up, don't muss it.
It is chiefly American and informal, and it is used most of hair — mussing somebody's hair is an affectionate gesture. Mussed as an adjective means rumpled. The British equivalents are ruffle and mess up. Note that muss is related to mess, and the two words share the sense of disorder without sharing the sense of dirt.
1verbmussTo make something untidy by ruffling it.
“Grandpa mussed my hair and told me how much I'd grown.”
muss is a ruffling.
Also accepted in games: tousle
Muss is related to mess, meaning disorder. American English kept it for ruffling while British English uses mess up or ruffle instead.
Related to mess (“disorder”).
Etymology from Wiktionary, CC BY-SA