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 thumb
          : Completely under control

Under the weather
          : Feel sick / depress

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

Under the table
          : Bribes / Pay money for illigal purposes

Up to you
          : Depend on you / rely on you