Origin: 1250–1300; Middle English < Old French < Late Latin dēfēnsa a forbidding, noun use of feminine of past participle of Latin dēfendere to defend; replacing Middle English defens < Anglo-French, Old French < Medieval Latin defēnsum (thing) forbidden, neuter past participle of Latin dēfendere
c.1300, from O.Fr. defens, from L. defensum "thing protected or forbidden," from neut. pp. of defendere "ward off, protect" (see defend). First used 1935 as a euphemism for "national military resources." Defense mechanism in psychology is from 1913.