A 'muts' in the Dutch language is a soft hat of any shape. Usually this refers to a beanie of some sort, but can include santa hats, the coonskin Davy Crockett, Soviet Cossacks ("The Loser") and even soft hats with a peak, ala "the Gavin DeGraw". There are many types of mutses.
However, muts is also used in the Dutch language as an insult after somebody does something dumb. (Possibly due to the fact that these types of hats can cover the eyes and ears). Muts is often preceeded by "dumb", as in "My word, you are a dumb muts."
Not to be mistaken for 'dumb mutt', although it shares a similar meaning. Muts is also a vague innuendo for the female genitalia, hence it being more commonly - but far from exclusively - used for females.