Etymology
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insidious (adj.)

1540s, from French insidieux "insidious" (15c.) or directly from Latin insidiosus "deceitful, cunning, artful, treacherous," from insidiae (plural) "plot, snare, ambush," from insidere "sit on, occupy," from in- "in" (from PIE root *en "in") + sedere "to sit," from PIE root *sed- (1) "to sit." Figurative, usually with a suggestion of lying in wait and the intent to entrap. Related: Insidiously; insidiousness.

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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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Rasputin 

acquired name (Russian, literally "debauchee") of Grigory Yefimovich Novykh (c. 1872-1916), mystic and faith healer who held sway over court of Nicholas II of Russia. His nickname is from his doctrine of "rebirth through sin," that true holy communion must be preceded by immersion in sin. His name has been used figuratively in English from 1937 for anyone felt to wield an insidious and corrupting influence.

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subtle (adj.)

c. 1300 (mid-13c. as a surname), sotil, "penetrating; ingenious; refined" (of the mind); "sophisticated, intricate, abstruse" (of arguments), from Old French sotil, soutil, subtil "adept, adroit; cunning, wise; detailed; well-crafted" (12c., Modern French subtil), from Latin subtilis "fine, thin, delicate, finely woven;" figuratively "precise, exact, accurate," in taste or judgment, "fine, keen," of style, "plain, simple, direct," from sub "under" (see sub-) + -tilis, from tela "web, net, warp of a fabric," from PIE root *teks- "to weave," also "to fabricate." According to Watkins, the notion is of the "thread passing under the warp" as the finest thread.

From early 14c. in reference to things, "of thin consistency;" in reference to craftsmen, "cunning, skilled, clever;" Depreciative sense "insidious, treacherously cunning; deceitful" is from mid-14c. Material senses of "not dense or viscous, light; pure; delicate, thin, slender; fine, consisting of small particles" are from late 14c. sotil wares were goods sold in powdered form or finely ground. Partially re-Latinized in spelling, and also by confusion with subtile.

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