U might stand for under or un-things.
Under snow - I don't remember what was under the November snowfall that broke the 1955 record. AKA ~ under the weather.
Underwater - comes with the thaw.
Underthings - unmentionable.
Unknown - a huge category.
Unwords: a delightful site full of unwords can be found here http://www.unwords.com/alpha/U/0.html or can be purchased here: The Unword Dictionary: 1,000 Words For Things You Didn't Think Had Words! (How America Speaks series)
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