The story
- Medieval Latin · borrowed fromratiō dēcidendī
- English todayratio decidendi
Borrowed from Medieval Latin ratiō dēcidendī, from ratiō (“reckoning, method, reasoning”) + dēcidendī, genitive singular of dēcidendus, gerund of dēcidere (“to settle”).
Etymology from Wiktionary, CC BY-SA 4.0.