The justaucorps is a semi-formal jacket that became popular in France between 1690 and 1710. It stayed in vogue for about a century, until French fashion began to adopt more Insular stylings. The justaucorps is best recognized in modern culture as the "pirate's jacket," as captains of privateer ships in the 17th and 18th centuries often wore them out to sea, and they are most frequently thus portrayed in cinema and television programming.
The word "justaucorps" is very visibly the compoundment of the French term "just au corps", which translates to English as "that which conforms to the body". Such a general name is only fitting (pun intended) because while French outergarments were usually of exceptional quality and measure, at the time, they were seldom designed to take the shape of their regularly portly employer. The justaucorps is very similar in general construction to the more modern frock coat; it is a skirted jacket which falls to just above the knees, tapering at the waist and holding tight to body with unpadded shoulders and narrow sleeves. It can be characterized by buttons running at the front closure from the throat nearly to the bottom, large, emboidered (but entirely functional) pockets, and huge cuffs which fold back from the mid-forearm to past the elbow. The skirts are pleated in the hind to increase volume, and have one large central vent to accomodate equestrian application, or a saber scabbard. Justaucorps can be made from most any fabric, ranging from coarse, cheap ragg to fine cashmere and merino wools, and were conversely made both as daily wear and special dress.
Whilst wearing his justaucorps, the 18th century French gentleman would have rather given his life than be seen without a gilet. "Gilet" is just the term for "waistcoat" in French, but at the time waistcoats were long, sleeved garments -- much different from the vests more commonly worn today.
References:
http://www.gentlemenoffortune.com/Justaucorps.htm
http://dictionary.reference.com/browse/justaucorps
http://costumebase.org/en/1700/glossar/justaucorps.html