poor as a churchmouse - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Having little or no wealth and few possessions, as in She's poor as a churchmouse, so you can't expect her to donate anything. The reason for this long-used simile is unclear, but most believe that, since churches are not known for storing food, a mouse inside one would fare poorly. It has survived such earlier phrases as poor as Job. [Second half of 1600s]