Ugly customer 
Dangerous person

Uncle Sam 
Refer to something from the US

Uncle Tom 
Person who are not updated to the current world.

Under age 
Below the legal age

Under cover 
Conceal / Hide

Under fire
Under the serious situation

Under the breath
Whisper / Talk in a very soft level

Under the nose
Did not recognize something which are actually very close

Under the hammer 
About to reach the final

Under the table
Bribes / Pay money for illigal purposes

Under the thumb
Completely under control

Under the weather
Feel sick / depress

Under your thumb
Under totally your control

Until the last dog is shot
Until the very last possible moment

Up a gumtree
In trouble. Very busy

Up in the air
Unclear situation

Upside down
Turn the top to the bottom and the bottom to the top

Up to speed
Update the progress

Up the wall
Very angry

Up to you
Depend on you / rely on you