nidus n.

"structure built by a bird or domestic fowl for the insulation and rearing of its young," Old English nest "bird's nest; snug retreat," also "young bird, brood," from Proto-Germanic *nistaz (source also of Middle Low German, Middle Dutch nest, German Nest; not found in Scandinavian or Gothic), from PIE *nizdo- (source also of Sanskrit nidah "resting place, nest," Latin nidus "nest," Old Church Slavonic gnezdo, Old Irish net, Welsh nyth, Breton nez "nest"), probably from *ni "down" + from PIE root *sed- (1) "to sit."

From c. 1200 of an animal or insect. Used since Middle English in reference to various accumulations of things, especially of diminishing sizes, each fitting within the next (such as a nest of drawers, early 18c.). Nest egg "retirement savings" is from 1700; it was originally "a real or artificial egg left in a nest to induce the hen to go on laying there" (nest ei, early 14c.), hence "something laid up as the beginning of a continued growth."

1610s, "shallow recess in a wall," from French niche "recess (for a dog), kennel" (14c.), perhaps from Italian nicchia "niche, nook," which is said to be from nicchio "seashell," itself said by Klein, Barnhart, etc. to be probably from Latin mitulus "mussel," but the change of -m- to -n- is not explained (Century Dictionary compares napkin from Latin mappa). Watkins suggests that the word is from an Old French noun derived from nichier "to nestle, nest, build a nest," via Gallo-Roman *nidicare from Latin nidus "nest" (see nidus), but that, too, has difficulties. The figurative sense is recorded by 1725. Biological use dates from 1927.

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."