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.