This is the 14004th most frequent German word.
gefundenes
"Gefundenes" means "found" or "something found" in English.
Here, 'gefundenes' acts as an adjective describing the noun 'Buch', implying that the book was found.
Ich habe ein gefundenes Buch im Park gelesen.
I read a found book in the park.
In this idiomatic expression, 'gefundenes' is part of 'gefundenes Fressen', meaning a suitable or ideal thing for something.
Das war ein gefundenes Fressen für die Gelegenheit.
That was a perfect opportunity for the occasion.
Here, 'gefundenes' describes 'Geschäft', indicating that the business was discovered or found.