sakkos
A richly decorated vestment worn by Orthodox bishops, instead of a priest's phelonion (chasuble in western church).
When in 1411 Emperor John VIII Palaeologos married a daughter of Vasilii II, Grand Prince of Muscovy, he sent Moscow a splendid specimen of the liturgical vestment known as a sakkos as a gift for Metropolitan Photios.
Where it comes from
From Byzantine Greek σάκκος (sákkos). Doublet of sac, saccus, sack, and saco.
Adapted from the Wiktionary entry sakkos (CC BY-SA 4.0).