ZAD,
crooked, like the letter Z; as, A meer Zad, used of any bandy-legg'd, crouch-back'd or deformed Person.
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Entry taken from 1736 Canting Dictionary, by Nathan Bailey.
Zad in the 1811 Vulgar Tongue
Also see examples in Canting Songs and Slang Rhymes collected by John S. Farmer (1896).