1789, "a sitting, a session," as of a learned society, originally in French contexts, from French séance "a sitting," from seoir "to sit," from Latin sedere "to sit" (from PIE root *sed- (1) "to sit"). Meaning "spiritualistic session in which intercourse is alleged to be held with ghosts of the dead" is recorded by 1845.

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.

1725, "adhering close to the surface," from Latin sessilis "pertaining to sitting, for sitting on," from sessus, past participle of sedere "to sit" (from PIE root *sed- (1) "to sit"). In botany, of leaves, etc., "attached without any apparent projecting support," from 1753. Meaning "sedentary," of certain animals fixed to one spot, is recorded by 1860.