Birthday suit

If you are in your birthday suit, you are naked.

Bit between your teeth

If you take or have the bit between your teeth, you take or have control of a situation. (Bit = piece of metal in a horse's mouth)

Bit part

If someone has a small or unimportant role in something, they have a bit part.

Bit player

A bit player has a small or unimportant role in something.

Bite off more than you can chew

If you bite off more than you can chew, you take on more responsibilities than you can manage. 'Don't bite off more than you can chew' is often used to advise people against agreeing to more than they can handle.

Bite someone's head off

If you bite someone's head off, you criticise them angrily.

Bite the bullet

If you have to bite the bullet, you have to accept or face something unpleasant because it cannot be avoided.

Bite the dust

This is a way of saying that somebody has died, especially if they are killed violently like a soldier in battle.

Bite your lip

If you have to bite your lip, you have to make a conscious effort not to react or to keep quiet about something that displeases you.

Bite your tongue

If you bite your tongue, you refrain from speaking because it is socially or otherwise better not to.

Bits and bobs

Bits and bobs are small, remnant articles and things- the same as 'odds and ends'.

Bitter end

If you do something to the bitter end, you do it to the very end, no matter how unsuccessful you are.

Bitter pill to swallow

A bitter pill to swallow is something that is hard to accept.

Black and blue

This means bruised, either physically or metaphorically.

Black and white

When it is very clear who or what is right and wrong, then the situation is black and white.

Black as Newgate's knocker

(UK) If things are as black as Newgate's knocker, they are very bad. Newgate was an infamous prison in England, so its door knocker meant trouble.

Black hole

If there is a black hole in financial accounts, money has disappeared.

Black sheep

Someone who is the black sheep doesn't fit into a group or family because their behaviour or character is not good enough.

Blackball

If you vote against allowing someone to be a member of an organisation or group, you are blackballing him or her.

Blank cheque

If you are given a blank cheque, you are allowed to use as much money as you need for a project.

Blank slate

A blank slate is something that hasn't been developed or described in any detail.

Bleeding edge

Similar to 'cutting edge', this implies a technology or process that is at the forefront or beyond current practices. However, because it is unproven, it is often dangerous to use (hence the 'bleeding').

Bleeding heart

A bleeding heart is a person who is excessively sympathetic towards other people.

Bless your pointy little head

This expression is used as to patronise someone, especially when they don't realise that they're not very clever.('Bless your pointes little head' is also used.)

Blessing in disguise

If some bad luck or misfortune ultimately results in something positive, it's a blessing in disguise.

Blind acceptance

If people accept thing blindly, they accept them without questioning them at all.

Blind as a bat

If you are in total darkness and can't see anything at all, you are as blind as a bat.

Blind leading the blind

When the blind are leading the blind, the people in charge of something don't know anything more than the people they are in charge of, when they should have greater knowledge.

Blind-sided

If you are blind-sided, an event with a negative impact takes you completely by surprise.

Blink of an eye

If something happens in the blink of an eye, it happens so fast it is almost impossible to notice it.

Blood and thunder

An emotional speech or performance is full of blood and thunder.

Blood from a turnip

It is impossible to get something from someone if they don't have it, just as you cannot get blood from a turnip.

Blood is thicker than water

This idiom means that family relationships are stronger than others.

Blood is worth bottling

(AU) If an Australian says to you "Your blood is worth bottling", he/she is complimenting or praising you for doing something or being someone very special.

Blood out of a stone

If something is like getting blood out of a stone, it is very difficult indeed.

Blood, sweat and tears

If something will take blood, sweat and tears, it will be very difficult and will require a lot of effort and sacrifice.

Blot your copybook

If you blot your copybook, you make a mistake or do something wrong that will negatively affect someone's opinion of you.

Blow a fuse

If you blow a fuse, you become uncontrollably angry.

Blow a gasket

If you blow a gasket, you get very angry.

Blow by blow

A blow-by-blow description gives every detail in sequence.

Blow hot and cold

If you blow hot and cold on an idea, your attitude and opinion keeps changing; one minute you are for it, the next you are against.

Blow me down

People say '(well,) blow me down' when you have just told them something surprising, shocking or unexpected. ('Blow me down with a feather' is also used.) 

Blow off steam

(USA) If you blow off steam, you express your anger or frustration.

Blow out of the water

If something, like an idea, is blown out of the water, it is destroyed or defeated comprehensively.

Blow smoke

(USA) If people blow smoke, they exaggerate or say things that are not true, usually to make themselves look better.

Blow the cobwebs away

If you blow the cobwebs away, you make sweeping changes to something to bring fresh views and ideas in.

Blow the whistle

If somebody blows the whistle on a plan, they report it to the authorities.

Blow your mind

Something that will blow your mind is something extraordinary that will amaze you beyond explanation.

Blow your own horn

If you blow your own horn, you boast about your achievements and abilities. ('Blow your own trumpet' is an alternative form.)

Blow your own trumpet

If someone blows their own trumpet, they boast about their talents and achievements.  ('Blow your own horn' is an alternative form.)

Blow your stack

If you blow your stack, you lose your temper.

Blow your top

If someone blows their top, they lose their temper.

Blue blood

Someone with blue blood is royalty.