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1. At the drop of a hat- instantly, without hesitating
e.g. Her daughter left their house at the drop of a hat and nobody has talked to her since. 

2. Ball is in one's court- it's one's turn to make a decision
I have told you my plan. Now, the ball is in your court. 

3. Cut corners- do something to save money
Their family business went bankrupt so they needed to cut corners. 

4. hear it through the grapevine- listen to rumors about a person or thing
I have heard through the grapevine that he just got divorced .

5. Jump on the bandwagon- join something or someone
When everyone seems to be doing it doesn't mean that you also have to jump on the bandwagon.