Green’s Dictionary of Slang

mur n.

rum.

1859, 1860		Hotten Dict. of Modern Sl. etc.
1873		Sl. Dict.
1882		Sydney Sl. Dict. (2 edn) 11: Mur - Rum.
1896		‘Thieves’ Sl.’ Gent.’s Mag. CCLXXXI Oct. 348: A ‘nettock of mur’ would denote that favourite liquid refreshment , a ‘quartern of rum’.
1921		N&Q 12 Ser. IX 347: Myrrh. Rum.
1953		J. Franklyn Cockney 297: Another word that has broken loose is mur, current among old soldiers.