Definition: producing or capable of producing offspring; prolific
Another word for “fertility.” It also means someone with great creative output.
Why I Looked It Up
I encountered the word in Cutting for Stone.
Their gazes were flat […] noting the gap between her first and second toe which everyone knows is the only reliable gauge of fecundity.
In this context, it means when a woman has reached child-bearing age (or, given the wording here, when she is capable of having sex).
I also found at roughly the same time in Extra Life in relation to the productivity of soil:
In the right circumstances, it could be capable of staggering fecundity…