Green’s Dictionary of Slang

dummy v.

(US Und.) to disguise.

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[US]C. Brown Manchild in the Promised Land (1969) 152: I used to be slow at rolling reefers and at dummying reefers.

In phrases

dummy up (v.)

(N.Z. prison) to lace with drugs or alcohol.

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[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 63/1: dummy up v. to lace with drugs or alcohol: ‘Dummy up her drink and she won’t remember anything tomorrow.