leave somebody holding the baby.
If someone is left holding the baby, they are made responsible for a problem that others don't want to deal with.
"When the angry customer started to complain, my colleague disappeared and left me holding the baby."
back-room boys
This term refers to people who do important work but have no contact with the public.
"Back-room boys don't always receive the credit they deserve for their work."
back-of-the-envelope calculation
This expression refers to a quick approximate calculation done informally, as on the back of an envelope.
"I don't need the exact amount. Just give me a back-of-the-envelope calculation."
back of beyond
An isolated place located far from any town is said to be in the back of beyond.
"It took us hours to find Jack's house. He lives in the back of beyond."
(like the) back of one's hand
If you know something like the back of your hand, you are very familiar with it and know it in detail.
"Of course I won't get lost. I know London like the back of my hand!"
back into shape
To get yourself back into shape, you need to take some exercise in order to become fit and healthy again.
"Eva decided she'd have to get back into shape before looking for a job."
back on your feet
If you are back on your feet, after an illness or an accident, you are physically healthy again.
"My grandmother had a bad 'flu but she's back on her feet again."
back to the salt mines
Saying that you have to go back to the salt mines is a humorous way of talking about returning to work, usually with some reluctance.
"We get two days off at Christmas and then it's back to the salt mines!"
back to square one
To say that someone is back to square one means that they have not succeeded in what they were trying to do, so they have to start again.
"When the plans were refused, it was back to square one for the architect."
back to the wall
If you have your back to the wall, you are in serious difficulty with very few options available.
"With his back to the wall, the supplier had to accept the deal."
back the wrong horse
If you back or bet on the wrong horse, for example the loser in a contest, match or election, you support the wrong person.
"When I voted for him I was convinced he would win, but I backed the wrong horse!"
backhanded compliment
A remark that appears to express admiration but could also be interpreted as an insult is called a backhanded compliment.
"He said my presentation was "surprisingly clear". How's that for a backhanded compliment! Does that mean that I am usually not clear ?"
backseat driver
A passenger in a car who gives unwanted advice to the driver is called a backseat driver.
"I can't stand backseat drivers like my mother-in-law!"
bad egg
Someone who is a bad egg is an untrustworthy person often involved in trouble whose company should be avoided.
"I don't want my son to be friends with Bobby Smith. Bobby's a bad egg."
bad hair day
Originating as a humorous comment about one's hair being unmanageable, this term had broadened to mean 'a day when everything seems to go wrong'.
"What's wrong with Jenny? Is she having a bad hair day?"
(in) bad shape
A person who is in bad shape is in poor physical condition.
"I really am in bad shape. I must do more exercise."