heads-up=a warning that sth is going to happen, usually so that you can prepare for it.可数

Joe told the boss’s assistant that he wanted the assistant to give him a heads-up when the boss was coming.

on the flip side=on the other hand

I didn’t get good grades when I was a student, but on the flip side, I had very joyful childhood(不可数）.

imperialism=a policy of extending a country’s power and influence through diplomacy or military force.帝国主义

In the formal dinner, Kim voiced his support for what he said was Russia’s fight against imperialism.

rule out=exclude someone or something as a possibility，PV，vt，排除

There seem to be no tangible agreements between these two leaders, but it doesn’t rule out the possibility that deals may have been done behind the scenes because the whole visit of Kim has been shrouded in secrecy.

stockpile= a large accumulated stock of goods or materials, especially one held in reserve for use at a time of shortage or other emergency.储备物资，可数

North Korea is believed to have a vast stockpile of munition.

volatile= liable（可能的，倾向于）to change rapidly and unpredictably, especially for the worse.不稳定的

The political situation in many African countries is now volatile.

discretionary=available for use at the discretion（自由决定的自由，不可数）of the user. 

This is a discretionary task, which means you can finish it in your own way.

brouhaha=commotion喧闹，不可数

There was a big brouhaha after the company announced its bankruptcy.

stretcher= a framework（结构）of two poles with a long piece of canvas（帆布，不可数）slung（悬挂，vt，原形是sling）between them，used for carrying sick, injured, or dead people.担架，可数；chopper直升机（informal），=helicopter 

We need a stretcher to carry the severely-injured player out of the football field and a chopper to move him to the closest hospital.