Green’s Dictionary of Slang

carl rosa n.

[rhy. sl.; ult. the popular Carl Rosa Operatic Society, founded in London in 1875]

a poser, anyone pretending to be something that they are not; thus the old carl rosa, a fraud.

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[UK]D. Powis Signs of Crime 177: Carl Rosa A poser. Someone who is not what he seems. The name refers to a well-known operatic society. Fraud or deceit is sometimes referred to as ‘the old Carl Rosa’.