waffle -
waistcoat -
wardrobe -
wart plaster -
Wellington boots, wellingtoned -
wheeze - whelk -
whinge, whinging -
windscreen -
wind [someone] up -
wing mirror -
wireless -
wizard -
wonky -
Wotcher!
waffle
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)
waistcoat
U.S.: vest. Note that this is not just the kind of vest worn with a formal
suit, but any kind of vest.
dragon blood dripped gently and silently onto [Hagrid's] waistcoat (OP20
wardrobe
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)
wart plaster
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)
The rusty cauldrons and old Wellington boots that usually littered the steps by the back door were gone (DH6)
thin legs now encased in Wellington boots (DH20)
wellingtoned legs crossed (DH20)
wheeze
A joke, especially a joke that has been repeated a great deal.
Often referred to in the phrase 'old wheeze'.
"What are Weasleys' Wizard Wheezes?" (GF5)
whelk
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.
whinge, whinging
To whinge is to complain in a whiny way, so 'whinging' = 'whining'.
Mr. H. Potter
The Cupboard Under the Stairs
4 Privet Drive
Little Whinging
Surrey
(PS3)
windscreen
(British edition only)
(referring to part of an automobile) windshield
wind [someone] up
To tease someone, especially by deliberately misleading the person about
something
(NSOED)
"And don't let James wind you up."
- (DH/e)
wing mirror
(referring to part of an automobile) a rear-view mirror projecting from
the side of a motor vehicle
(NSOED)
"I only forgot to look in the wing mirror, and let's face it, I can use a Supersensory Charm for that."
(DH/e)
Ron, after many nervous glances at her, had taken a small wooden wireless out of his rucksack and started to try to tune it (DH20)
wizard (adj.)
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)
"What are Weasleys' Wizard Wheezes?" (GF5)
wonky
Unsteady, shaky, unreliable. This might be used to describe something
that's so badly put together that it's about to fall down.
"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)
Wotcher!
A greeting, shortened form of "what cheer!" Also spelled
"Wotcha".
[Mainly London use]
