You asked for an adjective for greatly fruitful, but then you also mention the noun phrase, great bounty, and the Latin noun, frux.
Fruitfulness, bounty and being filled with goodness are all part of
the meaning of the Cornu Copiae, the horn of the goddess, Copia;
also called the Horn of Plenty or cornucopia in English.
Perhaps, this answer is obvious and overused in our culture, however.
Other goddesses like Fortuna and Pax are depicted holding the Cornu Copiae. Here you can see a statue of Tyche (Greek Τύχη), the counterpart of the Roman goddess, Fortuna, holding the horn of plenty:
Cornu Copiae corresponds to the Greek Κέρας Ἀμαλθείας, the horn of Amalthea (Greek: Αμαλθεια), the goat who fostered and nursed Zeus. He broke off one of her horns and the story goes that he then promised to fill it with good things to repair his mistake.
Lewis and Short gives explanation and ancient sources under the entry for cornu, cornus- the horn of an animal:
Cornu Copiae (less correctly, but freq. in late Lat., as one word,
Cornūcōpĭae , and twice Cornūcōpĭa , ae, f., Amm. 22, 9, 1; 25, 2, 3),
acc. to the fable, the horn of the goat Amalthea placed in heaven,
Greek Κέρας Ἀμαλθείας (v. Amalthea), the emblem of fruitfulness and
abundance, Plaut. Ps. 2, 3, 5; Gell. 14, 6, 2; cf. Hor. C. 1, 17, 16;
id. C. S. 60; id. Ep. 1, 12, 29; Ov. M. 9, 88.
Addendum: Tellus Mater or Terra Mater, Mother Earth, is also depicted holding the Cornu Copiae on her lap with children by her side.