cas·sock 
(kăsək)
(kăsək)n.
An ankle-length garment with a close-fitting waist and sleeves, worn by the clergy and others assisting in church services.
[French casaque, long military coat, from Middle French, probably originally meaning "Cossack coat" and ultimately (probably via Old Russian kozakŭ, free man, Cossack) and of Turkic origin; akin to Tatar qazaq, free man, adventurer, and Kazakh qazaq, Kazakh; see KAZAKH.]