Synonyms of 'muss'
to throw into disorder
She let the wind ruffle her hair.
If you rumple someone's hair, you move your hand backwards and forwards through it as your way of showing affection to them.
I leaned forward to rumple his hair.
Thesaurus for muss from the Collins English Thesaurus
To muss something, especially someone's hair, or to muss it up , means to make it untidy.