1590s, of assemblies, parliaments, etc., "remaining in one place" (contrasted to ambulatory), from French sédentaire (16c.) and directly from Latin sedentarius "sitting, remaining in one place," from sedentem (nominative sedens), present participle of sedere "to sit; occupy an official seat, preside; sit still, remain; be fixed or settled" (from PIE root *sed- (1) "to sit").
Of occupations, etc., "requiring much sitting," by c. 1600. In reference to persons, "accustomed to long sitting," 1660s, hence "inactive, not in the habit of exercise." Related: Sedentariness.