Katie Bell [Katherine]:
Etym: St. Katherine or Catherine of Alexandria was a scholarly woman who was imprisoned for converting the wife and soldiers of the emperor Maxentius. He is said to then have dispatched his philosophers to convince her by logical arguments to denounce her faith, only to find that she instead converted them. Later she was martyred, and a particular instrument of torture associated with this is known as "St. Katherine's Wheel". The etymology of this name is unknown.
Keeper: The player on a Quidditch team who attempts to keep the Quaffle from going through any of the goal hoops.
kelpie: A malicious water creature able to assume a number of shapes, but usually appearing as a horse, which takes delight in the drowning of passers-by.
Kent: The home of the Wailing Widow.
Kettleburn (PoA ch. 5): The former Care of Magical Creatures teacher.
Etym: Means pretty much what it looks like.
King's Cross: A major train station in London, and the point of origin for the Hogwarts Express.
knickerbocker glory: A confection similar to a large ice cream sundae with all the attendant toppings, sauces, etc., traditionally served in a very tall glass.
Knight Bus: A bus summonable by any wizard in dire need of transportation which can take them anywhere they want to go on land.
Knockturn Alley: The evil twin of Diagon Alley, location of Borgin and Burkes.
Etym: Say it out loud...
knotgrass: Polygonum aviculare, and one of the ingredients for the Polyjuice Potion. Herbal lore says an infusion of knotgrass is supposd to stunt one's growth.
Knut: 1/493 of a Galleon, 1/29 of a Sickle.
Etym: Not sure, maybe just a variation of nut, the coins being brownish and all.
Kwikspell: A correspondence course in basic magic that Filch apparently tried.