Back seat driver
Person who like to involve in other's business.

Back to square one
Back to the starting point again

Back to the wall
In a ready-to-fight situation due to no other way to escape.

Backup the wrong horse
Support the wrong person, team or side.

Bad Egg
Person who is different, in a negative way, from the group.

Bad hair day
A day which everything seem to go wrong.

Bag of bones
Very skinny person/ animal.

Bear fruit            
Succeed

Beggar on horseback
Poor person who acting arrogant when become rich or achieve only small thing.

Bent on
Already made decision, agreed on something

Beyond Question
Certainly, No doubt

Birds of a feather flock together
The same type of people or people who like to do the same thing

Blessing in disguise
Good thing that you never reconginize at the first place

Blow away
Kill by gun shot

Blow over
Already finish, forget

Bound to do
Likely to happen / Likely to do something

Bottom of the barrel
The worst part of the group / organization

Break a leg
Good luck

Break into tears
Suddenly yell out (either sad or happy)

Break the ice
Talking or doing things to release pressure or to make listeners relax

By and large
Generally, In all case.

By birth
Specific character attached to someone from their family or country