as poor as a church mouse字面意思是“穷得像教堂里的老鼠”。我们知道，教堂是教徒们进行宗教活动的地方，教徒们虔诚地作礼拜，没有人在这里吃东西。所以老鼠在教堂里根本找不到吃的东西。教堂里的老鼠连吃的东西都没有，当然是非常穷的。所以这个成语就比喻“一贫如洗”了。

【例句】
With his company doing bankrupt, he is now as poor as a church mouse.
随着他的公司倒闭破产，他目前已是一贫如洗。

Nobody could imagine that he was as poor as a church mouse when he first came to Hong Kong.
但没有人会想到，他初到香港时，只是个一文不名的穷小子。