Here is a list of idiomatic expressions built around words beginning with letter B.
Be left holding the baby
When you are left holding the baby you are given a responsibility that you do not want.
The back of beyond
If something happens at the back of beyond, it happens at a very remote place.
Get someone’s back up
To get someone’s back up is to annoy them.
Turn your back on
To turn your back on something is to ignore or reject it.
With your back to the wall / Up against the wall
When you stand with your back to the wall, you are in a very difficult situation.
Bend over backwards
To bend over backwards is to try your hardest to be helpful or fair.
Bail someone out
To bail someone out is to rescue them from a difficulty.
Rise to the bait
To rise to the bait is to react to temptation exactly as someone planned.
Be in the balance / hang in the balance
When something hangs in the balance, it is in an uncertain state.
On the ball
When you are on the ball, you are alert.
To play ball is to cooperate.
Start or set the ball rolling
To set the ball rolling is to make a start.
Have a ball
To have a ball is to have a very enjoyable time.
Be called to the bar
When you are called to the bar, you are allowed to practice as a barrister.
When you are behind bars, you are in prison.
With your bare hands
To do something with bare hands is to do it without using any tools or weapons.
Drive a hard bargain
To drive a hard bargain is to try hard for a deal in your favor.
Someone’s bark is worse than their bite
When someone’s bark is worse than their bite, they are not as dangerous as they seem to be.
Be barking up the wrong tree
When you are barking up the wrong tree you are doing or thinking something that is not correct.
Bear someone a grudge
To bear someone a grudge is to feel resentment against them.
Bear something in mind
To bear something in mind is to take something into account.
Beat about the bush
To beat about the bush is to discuss a matter without coming to the point.
Off the beaten track – isolated
At someone’s beck and calls
When you are at someone’s beck and calls you are ready to obey their orders.
Have a bee in your bonnet
When you have a bee in your bonnet, you are obsessed with something.
Below the belt
Unfair or against the rules
Tighten your belt
To tighten your belt is to cut your spending.
Under your belt
When you have something under your belt, you have achieved or acquired it.
Give someone a wide berth
To give someone a wide berth is to stay well away from them.
When you are beside yourself you are frantic with worry.
Get the best of
To get the best of someone is to overcome them.
Make the best of
To make the best of something is to get what limited advantage you can get from them.
To better yourself is to improve your position.
When you are better off, you are in a more favorable position.
Get the better off
To get the better off someone is to defeat them.
Bide your time
To bide your time is to wait patiently for a good opportunity.
Too big for your boots
When you are too big for your boots, you are too proud of yourself.
Fit the bill
If something fits the bills it is suitable for the purpose.
Be bound by
When you are bound by something you are hampered or restricted by it.
The birds and the bees
An informal expression used to refer to the basic facts about sex.
Bit by bit
When you do something bit by bit, you do it gradually.
Do your bit
To do your bit is to make a useful contribution.
When you are thrilled to bits, you are very much thrilled.
Bite the bullet
To bite the bullet is to make yourself do something difficult.
Bite the hand that feeds you
To bite the hand that feeds you is to do something with the objective of offending someone who has been helpful to you.
Bite off more than you can chew
To bite off more than you can chew is to take on more than you can handle.
Bite your tongue
To bite your tongue is to stop yourself from saying something.
Blaze a tail
To blaze a tail is to be the first to do something.
Blind someone with
To blind someone with something is to confuse or overwhelm them with something they cannot understand.
Turn a blind eye
To turn a blind eye is to pretend not to notice something.
Knock someone’s block off
To knock someone’s block off is to hit them on the head.
Be in your blood / run in your blood
When something is in your blood or runs in your blood, it is an integral part of your character.
Have blood on your hands
When you have blood on your hands, you are responsible for someone’s death.
Make someone’s blood boil
To make someone’s blood boil is to make them very angry.
Blot your copybook
To blot your copybook is to do things that would damage your good reputation.
Blow hot and cold
To blow hot and cold is to keep changing your reputation.
Blow someone’s mind
To blow someone’s mind is to impress them very strongly.
When a trouble or controversy blows over, it fades away without serious effects.
When things blow up, they explode.
Once in a blue moon
When something happens once in a blue moon, it happens very rarely.
When something happens out of the blue, it happens when you least expect it.
The blues are the feelings of sadness/depression.
Call someone’s bluff
To call someone’s bluff is to challenge them to do something because you believe that they won’t be able to do it.
Be in the same boat
When you are in the same boat, you are in the same difficult situation as other people.
Miss the boat
You miss the boat when you are too slow to take advantage of an opportunity.
Rock the boat
To rock the boat is to disturb an existing situation.