Fr: just(e) au corps=right on the body. Male top garment.
A short coat or long jacket worn over a waistcoat (gilet, in the beginning even over an actual jacket with sleeves) and breeches (culottes) since the late 17th century. It was used for everyday as well as formal and court wear, the difference being in the quality of the fabric and the amount of trimmings.
As the name implies, the justaucorps was tight-fitting, with narrow shoulders and sleeves, in its upper part. From the waist down, however, it flared into horsehair-stiffened skirts. The skirts were given additional width by a number of pleats in the back side seams, at the top end of which sat a button each. Other distinctive features of the justaucorps are pockets with huge, embroidered flaps, large buttonholes (of which many were purely decorative) picked out in embroidery, and large sleeve cuffs. The lower part of the centre back was left open to allow the rapier to stick out, and for the saddle.
The skirts were quite voluminous in the early years, worn over an almost knee-length waistcoat with a straight seam. While the skirt volume decreased, fewer and fewer buttons really worked - they had always been only partially closed - and the waistcoats became shorter and sleeve cuffs smaller. Towards the end of the 18th century, a standing collar developed where there had been none before, the lower front edges were cut to fall open from the waist, and the back pleats vanished.
Developed form the justaucorps was the habit (or frac) á la franšaise, which in turn became the ancestor of the tailcoat and the modern smoking.
Early justaucorps, 1690s
full-skirted justaucorps, 1710s
English coat with collar and short waistcoat, 1750s
standing collar and cut-away front edges, 1780s