Always a bridesmaid, never a bride It means the person is not able to fulfil their wishes and dreams, the get close to achieving what they want, like a bridesmaid being part of a wedding, but they never get to attain their desire.
Armchair critic Someone who readily offers advice but they never seem to show anything worthwhile in their action to prove that they themselves are capable of following what they are preaching.

Armchair quarterback Someone who offers advice, especially about football, but can't play the game themselves.
Arrow in the quiver A strategy or option that can be used to achieve an objective.

As one man When people do something as one man, it means they are in complete agreement and are doing it exactly at the same time.

At a loose end Having a lot of spare time but not knowing what to do with it.

At cross purposes When people are cross purposes, they have totally different aims and opinions.

At the coalface Working at the coalface means a person is dealing with real problems and issues rather than sitting in an office discussing things in a detached way.

At your wits' end To be very frustrated and agitated, not knowing what to do next.

Avowed intent A solemn promise made publicly to attempt to reach a certain goal.

Babe in the woods An innocent person or inexperienced person, who is like a child lost in the woods.

Baby boomer A baby boomer is someone born in the years after the end of the Second World War, a period when the population was growing very fast. This is an American idiom.

Back burner When something is given low priority, it is said to be on the back burner.

Back foot To be in a disadvantaged and defensive position.

Back the wrong horse Giving support to the losing side or the wrong person.

Back to back Directly one after another.

Back to square one Back to where one started.

Back to the drawing board To start again on a design or project after failing to get it right the first time.

Back to the salt mine To return to one's work/job, usually with some reluctance.

Back to the wall To be in a difficult situation, without any means of escape.

Backseat driver An annoying person who likes to give advice to others who are doing something, especially when the advice is unwanted.

Bad Apple People who are bad and can make others come under their influence.

Bad blood Unpleasant feelings or animosity between people, especially due to things that happened in the past.

Bad egg Someone who cannot be trusted.

Bad hair day A day when everything seems to go wrong and not the way you planned.

— Ahzam Ahmed