sitz-bath (n.)

"To be or remain in that posture in which the weight of the body rests upon the posteriors" [OED], Middle English sitten, from Old English sittan "occupy a seat, be seated, sit down, seat oneself; remain, continue; settle, encamp; lie in wait; besiege" (class V strong verb; past tense sæt, past participle seten), from Proto-Germanic *setjan (source also of Old Saxon sittian, Old Norse sitja, Danish sidde, Old Frisian sitta, Middle Dutch sitten, Dutch zitten, Old High German sizzan, German sitzen, Gothic sitan), from PIE root *sed- (1) "to sit."

With past tense sat (formerly also set, which is now restricted to dialect, and sate, now archaic); and past participle sat, formerly sitten. The meaning "to have a seat in a legislative assembly" is from late 14c.; in reference to the assembly, "to hold a session," from 1510s. The sense of "pose" for a portrait, etc., is by 1530s.

As short for babysit (v.) by 1966. The specific sense of "occupy a judicial seat" (Old English) is the notion in sit in judgement. The meaning "have a certain position or direction" is from c. 1200; of winds, "to blow from" (a certain quarter), 1590s, from the notion of "to be in."

To sit back "be inactive" is from 1943. To sit on one's hands originally was "withhold applause" (1926), later generalized to "do nothing" (by 1959). To sit around "be idle, do nothing" is by 1915, American English. To sit out "not take part, make oneself an exception" is from 1650s.

Old English bæð "an immersing of the body in water, mud, etc.," also "a quantity of water, etc., for bathing," from Proto-Germanic *badan (source also of Old Frisian beth, Old Saxon bath, Old Norse bað, Middle Dutch bat, German Bad), from PIE root *bhē- "to warm" + *-thuz, Germanic suffix indicating "act, process, condition" (as in birth, death). The etymological sense is of heating, not immersing.

The city in Somerset, England (Old English Baðun) was so called from its hot springs. Bath salts is attested from 1875 (Dr. Julius Braun, "On the Curative Effects of Baths and Waters"). Bath-house is from 1705; bath-towel is from 1958.

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."