Green’s Dictionary of Slang

(UK Und.) to murder.

1725		New Canting Dict. n.p.: hush very still, quiet, whence a Person is said to be Hush’d, in a Canting Sense, when he is murder’d, &c.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. n.p.: hushed murdered, &c.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1859		‘Scene in a London Flash-Panny’ Matsell Vocabulum 100: The cull that was hushed for his reader.
1862		E. de la Bédollière Londres et les Anglais 315/1: hush, [...] assassiner.