be on one's back 卧病
【讲解】
此处的back是名词，是"脊背"的意思,短语be on one's back原意为"仰卧"，转义指"躺在床上，无法起床"，在口语中引申为"生病"。
【对话】
A：Where's Peter? I haven't seen him for quite a few days.
A: 彼得在哪儿呢？我好几天没见着他了。
B: He's on his back now.
B：他卧床不起了。
A: I'm sorry to hear that. But he was all right last week.
A: 真不愿听到这个。但上周还好好的呢。
B: Yeah, he was caught in the rain the other day. He has got a very had cold.
B: 是呀，那天下了场雨，得了重感冒。
