Here is our list of American idioms that start with "U":

Uncle Sam: The United States government; Example: "One fourth of my salary goes to Uncle Sam."

Under construction:
Being built or repaired. Example: "This road has been under construction for six months."

Under fire:
Being attacked. Example: "The soldiers came under fire when they approached the city."

Under the table:
Illegally. Example: "Many illegal immigrants try to find work under the table."

Under the weather:
Sick. Ill. Example: "I'm feeling a bit under the weather today."

Up and about:
Recovered from an illness. Example: "It's nice to see you up and about. You must be feeling a lot better."

Up for grabs: Available (for the taking); Example: "The jackpot is still up for grabs."

Up in the air:
Uncertain. Example: "His future at this company is up in the air."

Up one's sleeve (as in "to have something up one's sleeve"): To have a secret agenda, to be up to something, to be planning something. Example: "I don't trust him. I just know he has something up his sleeve."

Upbeat (adj.): Positive. Example: "Brendon's always really upbeat."

Upper crust: Upper class. The richest, most important people in a certain society.

Uptight:
Conservative, nervous, nit-picky. "It's hard for me to feel at ease around uptight people."

(To be/feel) up to (doing something):
Capable of, fit for. Example: "Do you feel up to playing a game of tennis?"

(To be) up to no good: To be planning something bad. Example: "Jimmy's parents could tell that he was up to no good."

Up to one's neck in something:
Very much involved in something; to have a lot of something. Example: "Please don't give me any more socks as presents. I'm up to my neck in socks."

Up to par:
Meeting normal standards. Example: "The wine was nice, but it wasn't up to par with the excellent wine they normally have."

(To) use every trick in the book:
To use every method possible. Example: "He used every trick in the book to get her to go out on a date with him."