Green’s Dictionary of Slang

dummy adj.

worthless, disappointing.

1826

Pierce Egan’s Life in London 21 May 549/1: The Bonnifaces on the road-side appeared all upon the fret [...] anxiously upon the look-out to pick up a straggler, to buy a thimbleful of summut [...] it was a dummy thing altogether, as to the road,, [...] as flat as a pancake.

In phrases

dummy shuffle (n.)

(N.Z. prison) a disguised move.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 63/1: dummy shuffle n. a disguised move.