Frock Coat
Dictionary.com Unabridged (v 1.1) -
–noun
a man's close-fitting, knee-length coat, single-breasted or double-breasted and with a vent in the back.
[Origin: 1735–45]
Justacorps
Dictionary.com Unabridged (v 1.1) -
just·au·corps /ˈʒustəˌkɔr, -ˌkoʊr; Fr. ʒüstoʊˈkɔr/ Pronunciation Key - Show Spelled Pronunciation[zhoo-stuh-kawr, -kohr; Fr. zhyst-oh-kawr] Pronunciation Key - Show IPA Pronunciation
–noun, plural -corps /-ˌkɔr, -ˌkoʊr; Fr. -ˈkɔr/ Pronunciation Key - Show Spelled Pronunciation[-kawr, -kohr; Fr. -kawr] Pronunciation Key - Show IPA Pronunciation.
a fitted, knee-length coat, characterized by wide turned-back cuffs and stiff flared skirts, worn esp. by men in the 17th and 18th centuries.
Also called justicoat.
[Origin: 1650–60; < F juste au corps fitting to the body]
Either way we are talking about a long coat with skirts to the knees and NOTE the origin dates of the words..