The cassock, an item of Christian clerical clothing, is an ankle-length robe worn by clerics of the Roman Catholic Church, Eastern Orthodox Church, Anglican Church, Lutheran Church, some of the Oriental Orthodox churches and ministers and ordained officers of the Presbyterian and Reformed churches. "Ankle-length garment" is the literal meaning of the corresponding Latin term, vestis talaris. In the Western Christian tradition the cassock is generally close-fitting, but in the Eastern Orthodox Church and the Oriental Orthodox churches the outer cassock is quite loose.
The cassock derives historically from the tunic that in ancient Rome was worn underneath the toga and the chiton that was worn beneath the himation in ancient Greece.
The word "cassock" comes from Middle French "casaque", meaning a long coat. In turn, the old French word may come ultimately from Turkish "quzzak" (nomad, adventurer - the source of the word "Cossack"), an allusion to their typical riding coat, or from Persian کژاغند "kazhāgand" (padded garment) - کژ "kazh" (raw silk) + آغند "āgand" (stuffed).
In Ireland and in several other English-speaking countries, it is also known by the French-derived word soutane.
In the West, the cassock is little used today except for religious services; but in many countries it was the normal everyday wear of the clergy until the second half of the 20th century, when it was replaced even in those countries by a conventional suit, distinguished from lay dress by being generally black and by incorporating a clerical collar.
Read more about Cassock: Eastern Practice (Orthodox and Eastern Catholic), Non-clerical Seventeenth Century Garment, Cassocks in Popular Secular Culture