The dress worn on occasions of ceremony. If a man has no special costume, his “full dress” is a suit of black, open waiscoat, swallow-tailed coat, white neckcloth, and patent-leather boots or half-boots. Academicals are worn in the Universities and on official occasions; and full military dress is worn when an officer is on duty, at court, and at official fêtes, but otherwise, “evening dress” suffices.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Full Dress from Fact Monster: