A babe in arms is a very young child, or a person who is very young to be holding a position.
A babe in the woods is a naive, defenseless, young person.
(USA) 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.
If an issue is on the back burner, it is being given low priority.
(UK) If you are on your back foot, you are at a disadvantage and forced to be defensive of your position.
Something that's a back number is dated or out of fashion.
If you back the wrong horse, you give your support to the losing side in something.
If things happen back to back, they are directly one after another.
If you are back to square one, you have to start from the beginning again.
If you have to go back to the drawing board, you have to go back to the beginning and start something again.
If someone says they have to go back to the salt mine, they have to return to work.
If you have your back to the wall, you are in a difficult situation with very little room for man oeuvre.
A backseat driver is an annoying person who is fond of giving advice to the person performing a task or doing something, especially when the advice is either wrong or unwelcome.
A person who is bad and makes other bad is a bad apple.
If people feel hate because of things that happened in the past, there is bad blood between them.
A person who cannot be trusted is a bad egg. Good egg is the opposite.
If you're having a bad hair day, things are not going the way you would like or had planned.
(UK) When you are bad mouthing, you are saying negative things about someone or something.('Bad-mouth' and 'badmouth' are also used.)
If something's in bad shape, it's in bad condition. If a person's in bad shape, they are unfit or unhealthy.
If something leaves you with a bad taste in your mouth, you feel there is something wrong or bad about it.
Bad workers always blame their tools
"A bad worker always blames their tools" - If somebody does a job badly or loses in a game and claims that they were let down by their equipment, you can use this to imply that this was not the case.
If someone is a bag of bones, they are very underweight.
If someone is a bag of nerves, they are very worried or nervous.
A Baker's dozen is 13 rather than 12.
A person who is completely bald is as bald as a coot.
If the ball is in your court, it is up to you to make the next decision or step.
A ballpark figure is a rough or approximate number (guesstimate) to give a general idea of something, like a rough estimate for a cost, etc.
(USA) If you do something balls to the wall, you apply full acceleration or exertion.
Banana republic is a term used for small countries that are dependent on a single crop or resource and governed badly by a corrupt elite.
(UK) A banana skin is something that is an embarrassment or causes problems.
An area or an industry, profession, etc, where rules and laws are ignored or flouted is bandit territory.
A baptism of fire was a soldier's first experience of shooting. Any unpleasant experience undergone, usually where it is also a learning experience, is a baptism of fire.
A bar fly is a person who spends a lot of time drinking in different bars and pubs.
If you bare your heart to someone, you tell them your personal and private feelings. ('Bare your soul' is an alternative form of the idiom.)
A barefaced liar is one who displays no shame about lying even if they are exposed.
Someone who's bark is worse than their bite may well get angry and shout, but doesn't take action.
If you are barking up the wrong tree, it means that you have completely misunderstood something or are totally wrong.
This idiom means that someone is willing to get married.
(UK) A barrack-room lawyer is a person who gives opinions on things they are not qualified to speak about.
If someone's a barrel of laughs, they are always joking and you find them funny.
If something is a basket case, it is so bad that it cannot be helped.
If someone doesn't bat an eyelid, they don't react or show any emotion when surprised, shocked, etc.
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