A steal

If something is a steal, it costs much less than it is really worth.

A still tongue keeps a wise head

Wise people don't talk much.

A textbook case

A textbook case, it is a classic or common example of something.

A watched pot never boils

Some things work out in their own time, so being impatient and constantly checking will just make things seem longer.

A1

If something is A1, it is the very best or finest.

ABC

ABC means the basics of something- knowing the ABC of science, etc.

ABC

Meaning:basic terms or basic steps example:she don't know even ABC steps in dance.

Abide by a decision

If you abide by a decision, you accept it and comply with it, even though you might disagree with it.

Abject lesson

(India) An abject lesson serves as a warning to others. (In some varieties of English 'object lesson' is used.)

About as useful as a chocolate teapot

Someone or something that is of no practical use is about as useful as a chocolate teapot.

About face

If someone changes their mind completely, this is an about face. It can be used when companies, governments, etc, change their position on an issue.

Above board

If things are done above board, they are carried out in a legal and proper manner.

Above par

Better than average or normal

Above the salt

This means that something or someone has a high position.

Absence makes the heart grow fonder

This idiom means that when people are apart, their love grows stronger.

Accident waiting to happen

If something is an accident waiting to happen, there's definitely going to be an accident or it's bound to go wrong. ('Disaster waiting to happen' is also used.)

Ace in the hole

An ace in the hole is something other people are not aware of that can be used to your advantage when the time is right.

Ace up your sleeve

If you have an ace up your sleeve, you have something that will give you an advantage that other people don't know about.

Achilles' heel

A person's weak spot is their Achilles' heel.

Acid test

An acid test is something that proves whether something is good, effective, etc, or not.

Across the board

If something applies to everybody, it applies across the board

Across the ditch

(NZ) This idiom means on the other side of the Tasman Sea, used to refer to Australia or New Zealand depending on the speaker's location.

Across the pond

(UK) This idiom means on the other side of the Atlantic Ocean, used to refer to the US or the UK depending on the speaker's location.

Act of God

An act of God is something like an earthquake or floods that human beings cannot prevent or control.

Act of war

An act of war is a action that is either intended to start a war or that is interpreted as being sufficient cause for a war.

Actions speak louder than words

This idiom means that what people actually do is more important than what they say- people can promise things but then fail to deliver.

Adam's ale

(dated, humorous) water

Adam's apple

The Adam's apple is a bulge in the throat, mostly seen in men.

Add fuel to the fire

If people add fuel to the fire, they make a bad situation worse.

Add insult to injury

When people add insult to injury, they make a bad situation even worse.

After the watershed

The watershed is the time limit after which more controversial  subjects, bad language, etc, can be shown on TV in some countries, so if it's after the watershed, then discussions can be freer, franker and more controversial.

After your own heart

A person after your own heart thinks the same way as you.

Against the clock

If you do something against the clock, you are rushed and have very little time to do it.

Against the grain

If doing something goes against the grain, you're unwilling to do it because it contradicts what you believe in, but you have no real choice.

Age before beauty

When this idiom is used, it is a way of allowing an older person to do something first, though often in a slightly sarcastic way.

Agony aunt

An agony aunt is a newspaper columnist who gives advice to people having problems, especially personal ones.

Ahead of the curve

Similar to ahead of the pack, ahead of the curve literally refers to your position on the statistical bell curve, where the top of the curve represents the median, average result. By being ahead of the curve you represent the top percentile of results that either has the advanced skills or understanding that sets you apart.

Ahead of the pack

If you are ahead of the pack, you have made more progress than your rivals.

Ahead of time

If something happens ahead of time, it happens early or before the set time.

Air your dirty laundry in public

If you air your dirty laundry in public, you reveal aspects of your private life that should really remain private, by telling a secret, arguing in public, etc.

Albatross around your neck

An albatross around, or round, your neck is a problem resulting from something you did that stops you from being successful.

Alike as two peas

If people or things are as alike as two peas, they are identical.

Alive and kicking

If something is active and doing well, it is alive and kicking.  (It can be used for people too.)

All ages and stripes

A shorthand for expressing a diversity of folks in a group

All along

If you have known or suspected something all along, then you have felt this from the beginning.

All and sundry

This idiom is a way of emphasising 'all', like saying 'each and every one'.

All bark and no bite

When someone talks tough but really isn't, they are all bark and no bite.

All bets are off

(USA) If all bets are off, then agreements that have been made no longer apply.

All but

If someone all but does something, they almost do it, but don't manage to.

All cats are grey in the dark

Things are indistinguishable in the dark so appearances don't matter.('All cats are grey at night' is also used.)

All dressed up and nowhere to go

You're prepared for something that isn't going to happen.

All ears

If someone says they're all ears, they are very interested in hearing about something.

All eyes on me

If all eyes are on someone, then everyone is paying attention to them.

All fingers and thumbs

If you're all fingers and thumbs, you are too excited or clumsy to do something properly that requires manual dexterity. 'All thumbs' is an alternative form of the idiom.

All hat, no cattle

(USA) When someone talks big, but cannot back it up, they are all hat, no cattle.('Big hat, no cattle' is also used.)

All heart

Someone who is all heart is very kind and generous.