1640s, of textile fabric, "downy, fuzzy," later "flimsy, unsubstantial" (1660s), a word of unknown origin. One theory is that it is a corruption of Silesia, the German region, where thin linen or cotton fabric was made for export. Silesia, in reference to cloth, is attested in English from 1670s; and sleasie, sleazy as an abbreviated form is attested from late 17c., but OED finds the evidence to be against "any original connexion." The sense of "sordid, squalid" is from 1941. Related: Sleazily; sleaziness.
"Slesie linnen, so calld becaus brought from the province of Silesia, or as the Germans call it Schlesia, wher the capital city Breslaw is maintaind by this manufacture, which is the chief if not the only merchandize of that place." [from notes on Lansdowne manuscripts by antiquarian Bishop White Kennett (1660-1728), quoted in Halliwell]
A day is a more magnificent cloth than any muslin, the mechanism that makes it is infinitely cunninger, and you shall not conceal the sleazy, fraudulent, rotten hours you have slipped into the piece, nor fear that any honest thread, or straighter steel, or more inflexible shaft, will not testify in the web. [Emerson, "The Conduct of Life," 1860]
updated on December 22, 2022