Meaning of pay dirt

pay' dirt"

Pronunciation: [key]
  1. soil, gravel, or ore that can be mined profitably.
  2. any source of success or wealth; a fortunate discovery or profitable venture: After months of experimentation, the scientists finally hit pay dirt.
  3. See(def. 1).
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.