The term MESS UP, AS HAIR can be solved by two crossword solutions: MUSS (4 letters) and TOUSLE (6 letters). MUSS is a verb which means to disarrange or make untidy. It is often used in the context of hair, as it is a common way to describe the act of running one's fingers through their hair to make it look more disheveled. TOUSLE is also a verb which means to disarrange or make untidy, but it is usually used to describe the act of ruffling someone's hair. Both words are commonly used to describe the act of making someone's hair look messy.