Green’s Dictionary of Slang

(US) coffee.

1916		Day Book (Chicago) 8 Sept. 17/1: ‘Couple of dougnuts and a cup of coffee without cream’ [...] ‘Two submarines and a mug of murk — no cow!’.
1931		Irwin Amer. Tramp and Und. Sl. 75: In a railroad eating-house, [...] ‘a string of flats, plenty of pin grease and a tank of murk,’ would be merely an order for a plate of griddle or pancakes with plenty of butter, and a cup of coffee.
1936		H.W. Bentley ‘Linguistic Concoctions of the Soda Jerker’ in AS XI:1 44: MUG OF MURK. Cup of coffee without cream.
1949		Monteleone Criminal Sl. (rev. edn).