大声呼救的英文表达为"shout for help"或"cry for help."
这个表达的来源源于人们在紧急情况下大声呼救，以寻求帮助。
下面是一些例句：
She shouted for help when she saw the thief entering her house.
她看到小偷进入她的房子时大声呼救。
He cried for help when he was drowning in the lake.
他在湖里溺水时大声呼救。
The trapped miner shouted for help for hours before he was finally rescued.
被困的矿工数小时大声呼救，直到最终得到营救。