Accidentally on purpose

  If you do something intentionally, but pretend it was an accident, you do it

  'accidentally on purpose'.

  I accidentally-on-purpose erased his email address so I couldn't contact him again.

Add fuel to the flames

  If you add fuel to the flames, you say something that makes a difficult situation

  worse.

  He forgot their wedding anniversary, and his apologies only added fuel to the flames.

Answer the call of nature/nature's call

  When a person answers the call of nature, they go to the toilet.

  I had to get up in the middle of the night to answer the call of nature 

Be all ears

  To say that you are all ears means that you are listening very attentively.

  Of course I want to know - I'm all ears!

Badger someone into doing something

  If you badger someone into doing something, you persistently nag or pester them

  until you obtain what you want.

  Sophie badgered her parents into buying her a new computer. 

Bare your heart (or soul) to someone

  If you bare your heart or soul to someone, you reveal your innermost thoughts

  and feelings to them.

  John couldn't keep things to himself any longer.  He decided to bare his soul

  to his best friend.

Bark up the wrong tree.

  A person who barks up the wrong tree is doing the wrong thing, because their

  beliefs or ideas are incorrect or mistaken.

Beat a (hasty) retreat

  Someone who beats a (hasty) retreat runs away or goes back hurriedly to avoid a 

  dangerous or difficult situation.

  The thief beat a retreat as soon as he saw the security officer.

One's best bet

  The action most likely to succeed is called one's best bet.

  Your best bet would be to try calling him at home.

Blot one's copy book

  Someone who blots their copy-book does something to spoil their good record or

  reputation.

  He blotted his copy-book when he was arrested for speeding.

Bide your time 

   If you bide your time, you wait for a good opportunity to do something.

   He's not hesitating, he's just biding his time.  He's waiting for the price to drop.

Binge drinking

   This term refers to heavy drinking where large quantities of alcohol are drunk in a 

   short space of time, often among young people in rowdy groups

   Binge drinking is becoming a major problem in some European countries.

Bite the hand that feeds you

   If you bite the hand that feeds you, you are unfriendly or do harm to someone who 

   is kind to you.

Break every rule in the book.

   If you behave in a completely unacceptable way, you break every rule in the book.

   Our competitors obtained the contract by breaking every rule in the book.