Idioms and Phrases with grist for the mill
grist for the mill
Something that can be used to advantage, as in These seemingly useless data will be grist for the mill when he lodges a complaint. This expression alludes to grist, the amount of grain that can be ground at one time. [Late 1500s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.