"Kind of" and "sort of" are idiomatic adverbs that mean "somewhat" or "rather".
Also, sort of. Rather, somewhat, as in I'm kind of hungry, or The bird looked sort of like a sparrow. [Colloquial; c. 1800] This usage should not be confused with a kind of or a sort of, which are much older and refer to a borderline member of a given category (as in a kind of a shelter or a sort of a bluish color). Shakespeare had this usage in Two Gentlemen of Verona (3:1): “My master is a kind of a knave.” Also see of a kind.