U.F.O.
ugly duckling
uh-huh
um-hum
unbosom oneself
uncertain
uncle
under
under a bushel
under a cloud
under age
under arrest
under construction
under cover
under false colors
under fire
under lock and key
under one's belt
under one's breath
under one's hat
under one's heel
under one's nose
under one's own steam
under one's skin
under one's spell
under one's thumb
under one's wing
under orders
under pain of
under protest
understand
under the circumstances
under the collar
under the counter
under the hammer
under the nose of
under the sun
under the table
under the thumb of
under the weather
under the wire
underway
under wraps
unknown quantity
unseen
until all hours
until hell freezes over
until the last gun is fired
unturned
up against
up against it
up and about
up and at them
up-and-coming
up a stump
up a tree
up for grabs
up front(1)
up front(2)
up in arms
up in the air
up one's alley
up one's sleeve
upon oneself
upon one's head
upon the spot
upper
upper crust
upper hand
upper story
Upsadaisy!
ups and downs
upset the applecart
upside down
upstairs
up the creek
up tight
up to
up-to-date
up to no good
up to one's ears
up to one's neck
up to par
up to scratch
up to snuff
up to the chin in
up to the hilt
up to the last minute
up to the mark
up-to-the-minute
urban homesteading
use
used to(1)
used to(2)
used to be
use every trick in the book
use one's head
use up
utility room