Green’s Dictionary of Slang

(UK Und.) second-rate, fake.

1718		C. Hitchin Conduct of Receivers and Thief-Takers 6: To hang up a couple of Shim Sham Thieves, which he got little or nothing by.
1749		Newcastle Courant 18-25 Nov. 4/3: Is it for such Revard I rak’d de Stewe, / To catch shim-sham Nightingales for you?
1797		A.M. Bennett Beggar Girl (1813) I 22: To make a parcel of shim-sham ghosts and coffins, and such like blasphemies.