a bad apple
a person who is bad, rotten to the core, and brings others down.

– examples

He has always been a bad apple: stealing at school; trying to get his friends to take drugs; he even had a fight with his own mother.

Jonny is a bad apple, so stay away from him.

bad blood (between x and y)
deep hatred between people (so deep it is in their blood) that has lasted has for some time and started after a specific incident

– examples

In the play ‘Romeo and Juliet’, the bad blood between the lovers’ families helps bring about their deaths.

There’s quite a bit of bad blood between those two. It goes way back, so long they’ve probably forgotten what started it in the first place.

“Dave and Paul seem to fight a lot.”
“Yes, there has been some bad blood there since Dave stole Paul’s girl.”

(put something) on the back burner
to postpone or delay doing something because there is something important to be done first.

– examples

He has been planning to write a book for quite some time, but for now it is on the back burner because he’s busy with his new wife and child.

“Have you finished making your boat yet?”
“I’m afraid I had to put that on the back burner for the moment: I have a hundred things to do at work.”

back to square one
to have to start again

– examples

The boss told us our idea was stupid and he wouldn’t give us any money. I guess it’s back to square one.

He has been trying to get a date with her for months. Recently it looked like she would agree, but then he got drunk and made a fool of himself, so he’s back at square one.

back to the drawing board
have to go back to the planning stage

– examples

I took our project outline to the manager, but he said it would cost too much. I guess it’s back to the drawing board.

He really thought his new design would work, but on the very first day the prototype broke. He’ll have to go back to the drawing board.

back the wrong horse
to make the wrong choice; to pick the loser in a competition

– examples

He thought Dave was going to win the election, so put money into the campaign. Unfortunately it looks like he has backed the wrong horse in this case: Dave is losing by a mile.

When she was young she wanted to marry a rich guy. She had a choice of two: Dave and Paul. She chose Dave, but now it is Paul who has made a million, while Dave sits at home watching TV and eating chips. She definitely backed the wrong horse.

Before we put our money into one of these companies I want you to do some research: I don’t want us to back the wrong horse and end up feeling stupid.

(I have) a bone to pick with (you)
(usually spoken) to want to talk to someone about something they have done that has upset/ annoyed you

– examples

“Anne called him yesterday saying she had a bone to pick with him; apparently she heard he had been out at the pub when he had told her he was seeing his sister.”

“Hey, Dave, I have a bone to pick with you. What did you think you were doing telling my girlfriend I met my ex?”