"calm, quiet, placid," usually of persons or temperaments, 1660s, from Latin sedatus "composed, moderate, quiet, tranquil," past participle of sedare "to settle, make calm," causative of sedere "to sit" (from PIE root *sed- (1) "to sit"). Related: Sedately; sedateness (1640s).
"treat with sedatives," by 1945, a verb to go with the noun derivative of sedative (adj.). The word also existed 17c. in English with a sense of "make calm or quiet." Related: Sedated (by 1953 as an adjective, "under the influence of a sedative drug"); sedating.
Proto-Indo-European root meaning "to sit."
It forms all or part of: assess; assiduous; assiento; assize; banshee; beset; cathedra; cathedral; chair; cosset; dissident; dodecahedron; Eisteddfod; ephedra; ephedrine; ersatz; icosahedron; inset; insidious; nest; niche; nick (n.) "notch, groove, slit;" nidicolous; nidification; nidus; obsess; octahedron; piezo-; piezoelectric; polyhedron; possess; preside; reside; saddle; sanhedrim; seance; seat; sedan; sedate; (adj.) "calm, quiet;" sedative; sedentary; sederunt; sediment; see (n.) "throne of a bishop, archbishop, or pope;" sessile; session; set (v.); sett; settle (n.); settle (v.); siege; sit; sitz-bath; sitzkrieg; size; soil (n.1) "earth, dirt;" Somerset; soot; subside; subsidy; supersede; surcease; tanist; tetrahedron; Upanishad.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit a-sadat "sat down," sidati "sits," nidah "resting place, nest;" Old Persian hadis "abode;" Greek ezesthai "to sit," hedra "seat, chair, face of a geometric solid;" Latin sedere "to sit; occupy an official seat, preside; sit still, remain; be fixed or settled," nidus "nest;" Old Irish suide "seat, sitting," net "nest;" Welsh sedd "seat," eistedd "sitting," nyth "nest;" Old Church Slavonic sežda, sedeti "to sit," sedlo "saddle," gnezdo "nest;" Lithuanian sėdėti "to sit;" Russian sad "garden," Lithuanian sodinti "to plant;" Gothic sitan, Old English sittan "to sit."
1540s, "fixed, permanent," adjectival use of stayed, past participle of stay (v.). Meaning "sober, sedate" first recorded 1550s. As in Philip Sidney's justice staid, explained by Ruskin as "The desire of what is just, being stayed or restrained within the limits of what can be accomplished by just means." Related: Staidly.