Green’s Dictionary of Slang

a crooked person or thing.

1725		New Canting Dict. n.p.: zad crooked, like the Letter Z; as, A meer Zad, used of any bandy-legg’d, crouch-back’d, or deform’d Person.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. 1725].
1796		Grose Classical Dict. of the Vulgar Tongue (3rd edn) n.p.: Zad. Crooked like the letter Z. He is a mere zad, or perhaps zed; a description of a very crooked or deformed person.
1811		Lex. Balatronicum [as cit. 1796].
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1862		E. de la Bédollière Londres et les Anglais 319/2: zad, individu deformé, tourné en Z.