Idioms/U

If a thing or a person is an unknown quantity, not much is known about them.

If doing something is an uphill battle, an uphill struggle or an uphill task, it is difficult to do because of obstacles such as opposition from other people.

If you are one of the upper crust, you are a member of society's highest class.

If you have the upper hand, you have the advantage or you're in the stronger position in a contest or a conflict.

If you talk about "the ups and downs" in someone's life, or during a period of time, you talk about both the good times and the bad times.

If you "um and ah" you're having trouble deciding what to say, or you're having trouble telling somebody something.

If someone is under a cloud, they are suspected of having done something wrong.

You're under fire if you're being attacked by the media or cricitised by many people.

If something is under lock and key, it is kept in a very secure place.

If you are told that under no circumstances should you do something, you must never do it, no matter what happens.

If something is done under the table, it's done secretly, usually because it's illegal or unethical.

If you are under the weather, you're not feeling well.

If something is under wraps, it's being kept secret.

If you do something under your own steam, you do it without anybody's help.

If you're up a gum tree, you're in trouble or have a serious problem.

You can say something like a system or an organisation is up and running if it has started to operate.

If something is up for grabs, it's available for anyone who wants to try to get it.

If you are up in arms, you are angry about something that you think is unfair or wrong.

If something such as a plan to do something, or details of an agreement, are up in the air, they still haven't been decided or settled yet.

If someone is up to no good, they are doing something bad, or something wrong.

You can say something is up to scratch, or up to snuff, if it's as good as it should be, or as good as it needs to be.

If you're up to your neck in something, or up to your eyeballs in something, you've got too much of it and it's become a problem.

If someone's up-and-coming, they show signs of being successful in their profession.

If you upset the applecart, you do something that causes trouble or upsets someone's plans.

When we say something is user-friendly, we mean it is easy to use.