E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Isaac Taylor thinks this phrase means topers words, and derives fustian from fuste, Old French for a cask, whence fusty (tasting of the cask). It may be so, but we have numerous phrases derived from materials of dress applied to speech, as velvet, satin, silken, etc. The mother of Artaxerxes said, Those who address kings must use silken words. In French, faire patte de velour means to fatten with velvet words in order to seduce or win over.