Green’s Dictionary of Slang

Madame de Luce n.

deceptive talk.

1960		J. Franklyn Dict. of Rhy. Sl.
1972		Dodson & Saczek Dict. of Cockney Rhy. Sl.
1989		J. Morton Lowspeak 96: Madam – to tell lies; from the rhyming slang Madam de Luce = spruce (fib).
1992		R. Puxley Cockney Rabbit.