Green’s Dictionary of Slang

Nelson’s blood n.

rum.

1904		R. Free Seven Years Hard 122: Perhaps the reader may not be aware that [...] rum is ‘Nelson's blood’; that ‘gaffing’ is gambling ; ‘lumping,’ pawning.
1962		J. Wynnum Tar Dust 90: ‘What about a small keg of Nelson’s Blood,’ suggested Kate. [...] This was a special small quantity of overproof rum which the mess kept for signal occasions.