Green’s Dictionary of Slang

(US) crooked, corrupt; fraudulent.

1853		N.Y. Clipper 25 June 1/5: This match [a trotting race] is pronounced by those who know, to have turned out a regular ‘shyster’ affair, and . . . those who bet so largely on the Highland Maid are loud in their denunciations of the manner in which their favorite was ‘chiselled’ out of the purse.
c.1864		parody in Rakish Rhymer (1917) 122: These dam shyster f—kers, war widows and sly lovers, / Is agoing to run my business in the ground.
1870		J.K. Medbery Men and Mysteries of Wall Street 123: There are not a few individuals of the ‘shyster’ class who are ready to break their word, when they can shield themselves from prosecution.
1905		N.Y. Trib. 6 Aug. 41/2: Further light was also recently thrown on ‘shyster’ methods by the accusation of a criminal lawyer.
1910		L. Esson Woman Tamer in Ballades of Old Bohemia (1980) 66: We’re all taking the mugs down. One bloke, he says, does the trick with a silk hat on the Stock Exchange, and a shyster mine. We do it with a jemmy.
1934		S. Walker City Editor 112: [of newspaper photographers] There is a strong sense of honor, an undefined code which forbids shyster practices, even among this group of hard-boiled buccaneers.
1943		I. Wolfert Tucker’s People (1944) 67: That’s what it’ll be if you and your shyster crook lawyer talk till you’re blue in the face.
1946		H.A. Smith Rhubarb 106: And listen, you shyster shrimp, if you think —.
1987		C. Hiaasen Double Whammy (1990) 15: [...] you wouldn’t mind telling me why you’re moonlighting at that shyster insurance agency.