Green’s Dictionary of Slang

coffin nail n.1

[the assumption that drink, esp. in the outposts of the Empire, was a killer. Folk etymology erroneously links this nail with peg n.4 ; see also nail in one’s coffin under nail n.1 ]

a drink.

[UK](con. 1703) W.H. Ainsworth Jack Sheppard (1917) 8: The worst nail you can employ is a coffin nail. Gin Lane’s the nearest road to the churchyard.
[UK]Fun 4 Apr. 148: Silently they walked into the Gaiety bar just as though they were going to order a couple of coffins instead of only two more nails [F&H].
[UK]J. Walsh [perf. Vesta Tilley] Fairly Knocked the yankees in Chicago [lyrics] Said the barman, have a coffin and the people started laughing, / But I gave him one for chaffing then he created quite a row.
‘O. Henry’ ‘The Higher Abdication’ in Heart of the West 147: Say, sport, have you got a coffin nail on you?
[UK]‘William Juniper’ True Drunkard’s Delight.
[UK]D. Bolster Roll On My Twelve 97: Butch was the man who kept saying: ‘Well, here’s another nail in the old coffin.’.