Something very, particularly, or abundantly valuable or useful, especially that which has been discovered after a long or arduous search. Used especially in the phrase "hit/strike pay dirt." I was combing through books in the library for hours trying to find material for my thesis, when finally I hit pay dirt with an old collection of literary criticisms from the 1970s. I loved to search through my grandfather's attic when I was a kid, convinced that someday I would strike pay dirt.