Green’s Dictionary of Slang

eternity box n.

a coffin.

c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions .
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn).
1811		Lex. Balatronicum.
1812–82		B.M. Carew Life and Adventures.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
c.1850		Duncombe New and Improved Flash Dict.
1859		Matsell Vocabulum.
1864		London Standard 13 Dec. 3/2: ‘Eternity box,’ a coffin .
1881		Trumble Sl. Dict. (1890).
1895		Crowe Aus. Sl. Dict. 27: Eternity Box, a coffin.
1936		L. Pound ‘American Euphemisms for Dying’ in AS XI:3 201: Eternity box.