I. muss 1 /mʌs/ BrE AmE ( also muss up ) verb [transitive] informal especially American English
[ Date: 1800-1900 ; Origin: Probably from mess ]
to make something untidy, especially someone’s hair:
Briscoe reached down and mussed the boy’s hair.
II. muss 2 BrE AmE noun
no muss, no fuss used to say that something can be done easily and without problems – used humorously:
It works every time, no muss, no fuss.