The verb 'sit' is one of the oldest words in the English language — inherited in an unbroken line from Proto-Indo-European through Proto-Germanic to Old English to the present day. PIE *sed- (to sit) is among the best-attested PIE roots, with reflexes in virtually every branch of the family: Sanskrit 'sīdati' (sits), Greek 'hézesthai' (to sit) and 'hédra' (seat), Latin 'sedēre' (to sit), Old Irish 'saidid' (sits), Lithuanian 'sėdėti' (to sit), Old Church Slavonic 'sěděti' (to sit), and Gothic 'sitan' (to sit).
The Proto-Germanic form '*sitjaną' 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.
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 '*satjaną') meant 'to cause to sit, to place.' This intransitive/causative pair — where the transitive verb has a different vowel and means 'to make someone/something do the action' — is one of the oldest structural features
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 'sedēre': session, sediment, sedentary, sedate, president, reside, subside, obsess, assess, insidious, siege, see (a bishop's seat), possess, supersede. Through Latin 'sīdere' (to settle): side (debated