If something is above board, it's been done in a legal and honest way.
above the law
If someone is above the law, they are not subject to the laws of a society.
An Achilles' heel is a weakness that could result in failure.
across the board
If something is across the board, it relates to all without exception.
add fuel to the fire
If you add fuel to the fire, you do something to make a bad situation even worse.
add insult to injury
Someone adds insult to injury if they say or do something to upset you a second time, after you've already been upset somehow.
against all odds | against all the odds
If you do something against all odds, or against all the odds, you do it even though there were many problems and it didn't seem possible to do.
agree to differ | agree to disagree
If two people agree to differ, or agree to disagree, they accept that they have different opinions about something and stop trying to change each other's opinion.
ahead of the game
You are ahead of the game if you have an advantage over your competitors in any activity in which you try to do better than others, such as in business, academia, sports, etc.
all hell broke loose Informal
You can say "all hell broke loose" if a situation suddenly became violent or chaotic.
all the rage Informal
If something is all the rage, it's very popular or it's in fashion at the moment.
an acid test
An acid test is something that shows the true worth or value of something or someone.
an act of God Formal
Something like an earthquake or a tornado can be called an act of God.
an ax to grind (1)
If you have an ax to grind with someone, you have a problem with them, or a complaint against them, which you'd like to discuss.
an axe to grind (2)
If you have an axe to grind, you have a strong opinion about something and you express this opinion whenever you can.
another string to your bow
If you have another string to your bow, you have another way of making a living.
answer the call of nature
If you answer the call of nature, you go to the toilet.
around the clock
If something occurs around the clock, it goes on all day and all night.
as soon as possible | asap
If you do something as soon as possible (sometimes abbreviated to "asap"), you do it at the first possible opportunity.
asking for trouble
If someone is asking for trouble, they're doing something risky that could lead to a problem.
at a loose end
If you're at a loose end, you have nothing to do.
If you're at cross-purposes with someone, you think you're both talking about the same thing but you're actually talking about different things.
at loose ends
If you're at loose ends, you feel restless and unsettled because you don't have anything to do.
at sea | all at sea
If you're at sea, or all at sea, you're confused about something and not sure what to do.
at the drop of a hat
If you do something at the drop of a hat, you do it immediately, without preparation or planning.
at your wits' end
If you're at your wits' end, you're upset and frustrated because you've tried everything you can think of to solve a problem, and nothing has worked.