'Pay dirt' definitions:

Definition of 'pay dirt'

noun

Ore that yields a substantial profit to the miner

noun

A profitable success; "the inventor worked for years before hitting pay dirt"

From: GCIDE

Pay \Pay\, n.
1. Satisfaction; content. --Chaucer. [1913 Webster]
2. An equivalent or return for money due, goods purchased, or services performed; salary or wages for work or service; compensation; recompense; payment; hire; as, the pay of a clerk; the pay of a soldier. [1913 Webster]
Where only merit constant pay receives. --Pope. [1913 Webster]
There is neither pay nor plunder to be got. --L'Estrange. [1913 Webster]
Full pay, the whole amount of wages or salary; maximum pay; especially, the highest pay or allowance to civil or military officers of a certain rank, without deductions.
Half pay. See under Half.
Pay day, the day of settlement of accounts.
Pay dirt (Mining), earth which yields a profit to the miner. [Western U.S.]
Pay office, a place where payment is made.
Pay roll, (a) a roll or list of persons entitled to payment, with the amounts due; now usually one word, payroll. (b) the total sum of money which is paid to all employees on payday. [1913 Webster +PJC]

From: GCIDE

Pay dirt \Pay dirt\, Pay rock \Pay rock\, etc.
1. (Mining) Earth, rock, etc., which yields a profit to the miner. [Western U. S.] [Webster 1913 Suppl.]
2. a discovery, especially after an exploratory process, which yields a profit or sought-after benefit; -- used especially in the phrase "to hit pay dirt". [PJC]