Etymology
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stool (n.)

Old English stol "seat for one person," from Proto-Germanic *stōla- (source also of Old Frisian stol, Old Norse stoll, Old High German stuol, German Stuhl "seat," Gothic stols "high seat, throne"), from PIE *sta-lo-, locative of root *sta- "to stand, make or be firm."(source also of Lithuanian pa-stolas "stand," Old Church Slavonic stolu "stool").

Originally used of thrones (as in cynestol "royal seat, throne"); decline in sense began with adoption of chair (n.) from French, which relegated stool to small seats without arms or backs, then to "privy" (early 15c.) and thence to "bowel movement" (1530s).

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*sed- (1)

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."

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cathedra (n.)

"seat of a bishop in his church," 1829, Latin, literally "chair" (see cathedral).

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rocking (adj.)

"moving back and forth or to and fro," late 14c., rokking, present-participle adjective from rock (v.1). Of music, from 1949 (see rock (v.2)). Rocking-horse "wooden horse mounted on rockers for children" is recorded from 1724; rocking-chair "chair mounted on rockers" is from 1766.

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ladder-back (adj.)
1898 as a type of chair, from ladder (n.) + back (n.).
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lounging (n.)
1790, verbal noun from lounge (v.). Lounge chair is from 1841.
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recliner (n.)

1660s, "one who or that which reclines," agent noun from recline. From 1880 as a type of chair the back of which can be tilted as desired, earlier known as a reclining-chair (1831). Middle English had reclinatorie "a canopied bed or couch" (early 15c.).

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ex cathedra 
Latin, literally "from the (teacher's) chair," from ex "out of" (see ex-) + cathedra (see cathedral).
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dodecahedron (n.)

"solid having twelve faces," 1560s, from Greek dōdeka "twelve" (see dodeca-) + hedra "seat, base, chair, face of a geometric solid," from PIE root *sed- (1) "to sit." Related: Dodecahedral.

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cathedral (n.)

1580s, "church of a bishop," from phrase cathedral church (c. 1300), partially translating Late Latin ecclesia cathedralis "church of a bishop's seat," from Latin cathedra "an easy chair (principally used by ladies)," also metonymically, as in cathedrae molles "luxurious women;" also "a professor's chair;" from Greek kathedra "seat, bench," from kata "down" (see cata-) + hedra "seat, base, chair, face of a geometric solid," from PIE root *sed- (1) "to sit."

It was born an adjective, and attempts to force further adjectivization onto it in 17c. yielded cathedraical (1670s), cathedratic (1660s), cathedratical (1660s), after which the effort seems to have been given up.

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