from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. Having the pistil come to maturity before the stamens; protogynous.
from the GNU version of the Collaborative International Dictionary of English
- adj. Having the pistil come to maturity before the stamens; protogynous; -- opposed to
from The Century Dictionary and Cyclopedia
- In botany, exhibiting or characterized by proterogyny. See extract under proterandrous.
Ancient Greek earlier + woman, female. (Wiktionary)
Your view of its origin, from being proterogynous, seems to me very probable, especially as the females are generally the later-flowering plants.