From Longman Dictionary of Contemporary Englishmussmuss1 /mʌs/ (also muss up) verb [transitive] informal especially American English UNTIDYto make something untidy, especially someone’s hair Briscoe reached down and mussed the boy’s hair.→ See Verb table
Examples from the Corpus
muss• He never got his hair mussed.• Pre-Hugo, come to think of it, my hair never got mussed.• Grass long and mussed by wind.• The store man Alexander headed them, stepped toward the boy Earl Varney and mussed his hair.• The other was mussed, the bedspread pulled down and the pillows stacked against the headboard.• A warm breeze mussed up her wispy hair.mussmuss2 noun → no muss, no fussExamples from the Corpus
muss• No fuss, no muss and no need for professional advice.• What Dickie likes is no muss, no fuss.• No muss, no fuss, just good old displacement.Origin muss1 (1800-1900) Probably from mess 