Also, strike pay dirt. Make a valuable discovery or large profit, as in We've been researching the source of that quotation for a month and we finally hit pay dirt in the Library of Congress. This idiom, from the mid-1800s, refers to a miner's finding gold or other precious metals while sifting soil. By the late 1800s it had been transferred to other lucrative discoveries.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.