c. 1200, "mild, gentle, kind courteous," from Old French debonaire, from de bon' aire "of good race," originally used of hawks, hence, "thoroughbred" (opposite of French demalaire); aire here is perhaps from Latin ager "place, field" (from PIE root *agro- "field") on notion of "place of origin." Used in Middle English to mean "docile, courteous," it became obsolete and was revived with an altered sense of "pleasantly light-hearted and affable" (1680s). OED says it is "now a literary archaism, often assimilated in spelling to mod.F. débonnaire." Related: Debonairly.