1250-1300;Middle English < Old French < Late Latindēfēnsa a forbidding, noun use of feminine of past participle of Latindēfendere to defend; replacing Middle Englishdefens < Anglo-French,Old French < Medieval Latindefēnsum (thing) forbidden, neuter past participle of Latindēfendere
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.