Origins
The verb 'sit' is one of the oldest words in the English language β inherited in an unbroken line from Proto-Indo-European through Proto-Germanic to Old English to the present day.βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ PIE *sed- (to sit) is among the best-attested PIE roots, with reflexes in virtually every branch of the family: Sanskrit 'sΔ«dati' (sits), Greek 'hΓ©zesthai' (to sit) and 'hΓ©dra' (seat), Latin 'sedΔre' (to sit), Old Irish 'saidid' (sits), Lithuanian 'sΔdΔti' (to sit), Old Church Slavonic 'sΔdΔti' (to sit), and Gothic 'sitan' (to sit).
The Proto-Germanic form '*sitjanΔ ' produced Old English 'sittan,' Middle English 'sitten,' and modern 'sit.' The verb has been remarkably stable over more than a millennium of English usage, with only regular phonological changes affecting its form.
Proto-Indo-European Roots
The relationship between 'sit' and 'set' preserves an ancient grammatical mechanism. In Proto-Indo-European and Proto-Germanic, causative verbs were formed from basic verbs through vowel alternation (ablaut). 'Sit' (PIE *sed-) was the basic intransitive verb meaning 'to be in a seated position.' The causative 'set' (from Proto-Germanic '*satjanΔ ') meant 'to cause to sit, to place.' This intransitive/causative pair β where the transitive verb has a different vowel and means 'to make someone/something do the action' β is one of the oldest structural features of the Germanic languages. The same pattern appears in 'lie/lay' (to be lying / to cause to lie), 'rise/raise' (to go up / to cause to go up), and 'fall/fell' (to drop / to cause to drop).
The PIE root *sed- generated what may be the largest single word family in English when all branches are counted. Through Germanic: sit, set, seat, settle, saddle, nest. Through Latin 'sedΔre': session, sediment, sedentary, sedate, president, reside, subside, obsess, assess, insidious, siege, see (a bishop's seat), possess, supersede. Through Latin 'sΔ«dere' (to settle): side (debated), consider (originally 'to observe the stars,' perhaps from 'sitting with' the stars). Through Greek 'hΓ©dra': cathedral (from 'cathedra,' a chair), polyhedron, tetrahedron. The sheer number of English words traceable to this single PIE root makes *sed- one of the most productive etymological sources in the language.