E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Full fig. Full dress. A corruption of the Italian in fiocchi (in gala costume). It was derived from the tassels with which horses were ornamented in state processions. Thus we read in Miss Knights Autobiography, The Popes throne was set out for mass, and the whole building was in perfect fiocchi (in full fig). Another etymology has been suggested by a correspondent in Notes and Queries, that it is taken from the word full fig. (figure) in fashion books.
The Speaker sits at one end all in full fig, with a clerk at the table below.Trollope: West Indies, chap. ix. p. 101.