I heard the word rheithgor (/ˈr̩əiθgɔr/) on Radio Cymru this morning in the context of a report on a trial, and guessed that it meant ‘jury’. The second element, gor, comes from côr (/koːr/) (choir, circle), and the first element, rheith, appears in such words as rheithfawr (greatly just), rheithiad (regulation), rheithio (to fix a law), rheithiol (established as law), and also in rheitheg (rhetoric) and rheithegydd (rhetorician). So rheith seems to have something to do with justice and law and a rheithgor could be a ‘law choir’.
Another Welsh word that includes the element côr is pwyllgor /ˈpʊɨɬgɔr/ (committee) – the pwyll part means ‘discretion; steadiness’, and a related word, pwyllo, means ‘to steady, consider, reason, reflect’ – things that committees might do.
The English word jury comes from the Anglo-Norman jure(e), from the Old French jurée (oath, juridical inquiry, inquest), from the medieval Latin jūrāta, from iūrāre (to swear), from iūs (law, duty), from the Proto-Indo-European *yAus- (ritual purity; supreme justice), which is also the root of just and justice.
The English word choir comes from the Middle English quer(e), from the Old French cuer (church choir), from the Latin chorus (a company of dancers, dance; company, band) from the Greek χορός (dance, company of dancers or singers). Chorus comes from the same root.