Green’s Dictionary of Slang

Mahometan gruel n.

coffee.

1698		N. Ward London Spy I 15: My Friend and I pay’d for our Mahometan-Gruel, and away we came.
1772		R. Bradley Virtue & Use of Coffee 12: Mahometan gruel [...] black as Soot and tasting not unlike it.
c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Mahometan Gruel Coffee, because formerly chiefly used by the Turks.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1855		(ref. to 18C) Leicester Jrnl 7 Dec. 4/1: Let us call for what, in the lang of the day, was termed our ‘Mahometan gruel,’ or ‘Turkish sobriety’.
1862		E. de la Bédollière Londres et les Anglais 316/1: mahometan gruel, café.