kind

before 900; Middle English kinde, Old English gecynd nature, race, origin; cognate with Old Norse kyndi, Old High German kikunt, Latin gēns (genitive gentis); see kin

The phrase these (or those ) kind of, followed by a plural noun ( these kind of flowers; those kind of shoes ) is frequently condemned as ungrammatical because it is said to combine a plural demonstrative ( these; those ) with a singular noun, kind. Historically, kind is an unchanged or unmarked plural noun like deer, folk, sheep, and swine, and the construction these kind of is an old one, occurring in the writings of Shakespeare, Swift, Jane Austen, and, in modern times, Jimmy Carter and Winston Churchill. Kind has also developed the plural kinds, evidently because of the feeling that the old pattern was incorrect. These kind of nevertheless persists in use, especially in less formal speech and writing. In edited, more formal prose, this kind of and these kinds of are more common. Sort of has been influenced by the use of kind as an unchanged plural: these sort of books. This construction too is often considered incorrect and appears mainly in less formal speech and writing.
Kind (or sort ) of as an adverbial modifier meaning “somewhat” occurs in informal speech and writing: Sales have been kind (or sort ) of slow these last few weeks.