Green’s Dictionary of Slang

crooked, fake, spurious, counterfeit.

1959		J. Gosling Ghost Squad 25: Thieves’ argot, spoken properly, is a foreign language which needs to be learned [...] Among the words and phrases derived from rhyming slang are: [...] ‘Jekyll’ is snide (crooked) from Jekyll &Hyde.
1984		Partridge DSUE (8th edn) 615: [...] since ca. 1920.
2000		J.J. Connolly Layer Cake 87: He’d worked his way up from kiting, laying down jeckle paper with a cheque card. [Ibid.] 100: We’d be here all night while they acted the fool did their jeckle original-gangster impression for our benefit.
2011		J.J. Connolly Viva La Madness 63: The guy took a polite look at my Jekyll passport and waved me through.