1580s, from Late Hebrew sanhedrin (gedola) "(great) council," from Greek synedrion "assembly, council," literally "sitting together," from syn- "together" (see syn-) + hedra "seat" (see cathedral). Abolished at the destruction of Jerusalem, C.E. 70. The proper form is sanhedrin; the error began as a false correction when the Greek word was taken into Mishanic Hebrew, where -in is a form of the plural suffix of which -im is the more exact form.
