if you say that something will happen when hell freezes over, you mean that it will never happen.

Taxes will be abolished when hell freezes over.

• babe in the woods
• baby blues
• baby boomer
• back and forth
• back away
• back away from
• back away from something
• back-door
• back down
• back-handed compliment
• back in circulation
• backing and filling
• back in the game
• back in the saddle
• back into a corner
• back off
• back out
• back out of
• back out of something
• backroom boy
• backseat driver
• back someone into a corner
• back the wrong horse
• back to back
• back-to-back