What is the meaning of Muss?

To rumple, tousle or make (something) untidy.

In typical usage, the direct object of this verb is almost always the hair on an individual's head, especially but not exclusively the shorter hair of a man or boy. Less frequently, it can refer to other intimate appurtenances such as clothing, bedding, mascara or other makeup on an individual's face, or a collection of belongings that previously had been deliberately arranged or put in order. In these cases, the phrasal verb construction muss up is often used instead of muss. In most other contexts, mess (or mess up) would be more idiomatic.

A mess (disagreeable mixture or confusion of things; disorder)

A scramble, as when small objects are thrown down, to be taken by those who can seize them; a confused struggle.

A term of endearment.

first/third-person singular present of müssen

first/second/third-person singular present indicative of mussen

Source: wiktionary.org