Up to (a certain time)
Until, as far as a certain time, approaching a certain time.Up to last week I had never been inside a bowling alley.
Up to (a certain amount or number)
Until, as far as a certain amount or number, approaching a certain amount or number.There were probably up to thirty people at the meeting.
Up the river with no paddle
In trouble and unable to do anything about it.We were up the river with no paddle when we ran out of money on our vacation.
Up the creek with no paddle
In trouble and unable to do anything about it.I think that we are up the creek with no paddle now that our car has no gasoline.
Until hell freezes over
Forever.My friend said that he would not talk to his girlfriend again until hell freezes over.
Until all hours (of the day or night)
Until very late.We stayed up until all hours playing cards.
Unknown quantity
A person or thing which nobody knows much about.The new mayor is an unknown quantity and nobody knows what to expect.
Unfamiliar territory
An area of knowledge unknown to the speaker.Trying to pilot an airplane was unfamiliar territory for the flight attendant.
Unearthly hour
Absurdly early or inconvenient.We got up at an unearthly hour this morning so we could go camping.
Under wraps
Not allowed to be seen until the right time, in secrecy.The new car was still under wraps when the car show started.