Relating to or intended for defense; protective against harm or attack (archaic variant of defensive).
From defence/defense + -ative (suffix meaning 'relating to' or 'tending to'). This is an older or variant spelling of 'defensive,' preserving Latin roots more literally.
This archaic spelling 'defensative' shows how English once had more '-ative' words (like 'talkative' or 'creative'), but for this concept, 'defensive' won the spelling wars and 'defensative' mostly disappeared from use.
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