as poor as a church mouse字面意思是「窮得像教堂裡的老鼠」。我們知道，教堂是教徒們進行宗教活動的地方，教徒們虔誠地作禮拜，沒有人在這裡吃東西。所以老鼠在教堂裡根本找不到吃的東西。教堂裡的老鼠連吃的東西都沒有，當然是非常窮的。所以這個成語就比喻「一貧如洗」了。
as poor as a church mouse常在動詞be, look後做表語或在leave sb後做賓語補足語。比如：
You want to borrow money from him? He is as poor as a church mouse.
你想向他借錢？他窮得象教堂裡的老鼠。
Mrs. Johnson was as poor as a church mouse.
詹森夫人一貧如洗。
Being poor as church mice is all very well when you’re young, but Sam is nearly thirty now and ought to be looking for a better job.
年輕時沒有錢還無所謂，可山姆都快三十歲了，他是該找份好點的工作了。