Wash out
If an event is washed out, it's stopped because of rain.
wash up (1) 
If you wash up, you wash dirty dishes and cooking utensils.
wash up (2) 
If you wash up, you wash your hands.
Waste away
If you waste away, you gradually become thinner and weaker, usually because of an illness.
Watch out
If you tell someone to watch out, you tell them to be careful or warn them of a danger.
Watch out for
If someone tells you to watch out for something, they're warning you about it.
water down (1)
to add water to a drink to make it less strong if it's an alcoholic drink or less thick if it's a juice or a thick shake
water down (2)
to change something like the lyrics of a song or the dialogue in a movie to make it less offensive or less likely to upset people
Wean off
to gradually break a bad habit by doing it less and less
Wear down
to gradually make someone lose their strength and vitality
Wear in
If you wear in something like a pair of shoes, you wear them for short periods until they fit properly and feel comfortable.