defensive (adj.)

c. 1400, "serving to defend, proper for defense; of the nature of defense," from Old French defensif (14c., Modern French défensif) and directly from Medieval Latin defensivus, from defens-, past participle stem of Latin defendere (see defend). Of persons, "alert to reject criticism," from 1919. Related: Defensively; defensiveness.

As a noun, "that which defends or serves for defense," c. 1400, originally of medicines. Meaning "posture or attitude of defense" is from c. 1600.

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Definitions of defensive from WordNet
defensive (adj.)
intended or appropriate for defending against or deterring aggression or attack;
a defensive stance
defensive weapons
defensive (adj.)
attempting to justify or defend in speech or writing;
Synonyms: justificative / justificatory
defensive (n.)
an attitude of defensiveness (especially in the phrase `on the defensive');
Synonyms: defensive attitude