Originally (when referring to a small dog) to yap or bark - the word "waff"'s sound imitates a puppy's bark (NSOED). When referring to the noises people make, the word has come to mean empty or aimless talk or writing, and can be used as a noun to describe the sense-free noises themselves or as a verb to mean the act of producing them.
"Yes, there was a commotion outside the door and it flew open, and there was that rather uncouth barman standing with Snape, who was waffling about having come the wrong way up the stairs..."
- Professor Trelawney (HBP25)
A large piece of furniture with a door, used for storing clothes and fitted with rails, shelves, hooks and the like, sometimes with a mirror on the inside of the door. Wardrobes tend to be used in places that don't have built-in closets.
If this word was completely unfamiliar to you, please read The Lion, the Witch and the Wardrobe by C.S. Lewis.
...the end of the room, where there was nothing except an old wardrobe in which the teachers kept their spare robes (PA7)
A plaster that is medicated to treat warts.
wellington boots, wellingtoned
Rubber boots; rain boots. The adjective "wellingtoned" is used to describe someone wearing such boots.
Named for the first Duke of Wellington. In the U.S. editions "Wellington" is capitalized to reflect that it is a proper name, but it is not capitalized in the U.K. editions.
one of the Wellington boots that lay scattered around the door (GF5)
thin legs now encased in Wellington boots (DH20)
wellingtoned legs crossed (DH20)
A type of marine mollusc with a spiral shell, usually eaten with vinegar. The flesh is usually scooped out with a pin, so it's not the sort of thing a sophisticated lady would eat. This indicates that Marge is rather common in her tastes.
wind [someone] up
To tease someone, especially by deliberately misleading the person about something (NSOED)
"And don't let James wind you up."
(referring to part of an automobile) a rear-view mirror projecting from the side of a motor vehicle (NSOED)
Apart from the obvious (and in Muggle English, archaic) meaning of "associated with wizards or wizardry", this is also a British slang term for "excellent." (NSOED)
"They wouldn't look twice at him if he couldn't do that Wonky Faint thing -"
"Wronski Feint," said Harry, through gritted teeth.
- this really was too bad of Hermione, since the Wronski Feint is the very opposite of 'wonky' in terms of the dexterity needed to pull it off (GF19)