A fruitful one signifies spiritual good from charity.

(References: Apocalypse Explained 357)

533. Since nothing further is mentioned about Lamech apart from the fact that be beget sons and daughters, who are the conceptions and births of such a Church, there is no need to pause any longer over him. What these births, or sons and daughters, were, becomes clear from the Church, for the character of a Church determines that of its offspring. Both those Churches called Methuselah and Lamech breathed their last just before the flood.