To muss means to make something messy or disarranged, especially hair or clothing. When you wake up in the morning, your hair is often mussed from tossing and turning in your sleep. A parent might playfully muss a child's hair by rubbing their hand over it. A strong wind might muss your carefully combed hair right before a school photo.

The word suggests a gentle or casual kind of messiness, not serious damage or destruction. You wouldn't say a tornado mussed a building (you'd say it destroyed it), but you would say the wind mussed someone's outfit. When something gets mussed up, it becomes rumpled, tousled, or disheveled, but it can usually be fixed pretty easily with a comb or by straightening your clothes.

You might hear someone say “don't muss the sheets” before guests arrive, meaning keep the bed looking neat and smooth. The word has a slightly old-fashioned feel to it, but it perfectly captures that in-between state when something isn't ruined but definitely isn't tidy anymore either.