1)Achilles hills-If you have any weakness then it is your achilles hills and that weakness is very fatal that weakness can cause you lot of harm so never show your weakness to anyone because you might don't know how they are going to use it. 

2)Add fuel to fire-If there is a problem then you start making it worse then you are adding fuel to fire. Many times in offices when employees do politics to a new comer then they are adding fuel to fire which they should not be doing Infact they must make new comers feel easy because at some point everyone was new. 

3)An arm chair job-When you get a good income job with lot of comfort in it then you got an arm chair job. There are many manager or directors who do nothing just complaining and criticizing their employees but they get a good salary. Which make other junior envious of him but at some point they have also done really good job.

4) An axe to grind - when you do something for some selfish reasons where you use other people to excel in your life and this may give you profit but at some point you have to pay for it. because what you do, you get paid for it later. because every action has equal and opposite reaction. 

5)An iron will-In this ruthless world if you are not strong then people will eat you up so in order to get what you want you have to iron will because if you are not strong then you will not be able to survive this harsh world so get up and don't give up whether you face any problems in your life never get disappointed because life test those people who are warriors and one who get defeated they either suicide or suffer. 

6)An old flame - When you are in a relationship with a lady in the past but you are no longer talking to her and due to that you can mention your old past love with this phrase. 

7)an old head on young shoulders - when some is very young but due to responsibilities and past experiences that young boy has learnt so many things from setbacks that he is now become expert in handling with the problem. you must have seen many person with this where peole are very young to their age