Paydirt started as miners' talk: dirt, meaning earth, that has enough metal in it to pay you for digging. If you hit paydirt or strike paydirt, you have found the good earth — and then, by a stretch everyone now uses, you have found anything that suddenly pays off: a useful fact, a good idea, a deal, a bit of luck that makes you rich or successful. The sense learners meet is almost always this second one. Though actually English does not really use paydirt as a verb on its own. People say hit paydirt or strike paydirt. The tidy gloss 'to make a valuable discovery' is the meaning of that whole phrase, not of a verb paydirt. The word feels American, informal, and a little old and colourful, like a Western. You will also see it written pay dirt, as two words; same thing.