Decisive

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early 17th century: from French décisif, -ive, from medieval Latin decisivus, from decis- ‘determined’, from the verb decidere (see decide).

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From Middle French décisif

etymonline

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decisive (adj.)

1610s, "having the quality or power of determining," from Medieval Latin decisivus, from Latin decis-, past participle stem of decidere "to cut off; decide" (see decide). Meaning "marked by prompt determination" is from 1736. Compare decided. Related: Decisively; decisiveness.