Green’s Dictionary of Slang

(US) very strong drink, usu. alcoholic but sometimes coffee.

a.1859		Spirit of the Times (N.Y.) n.p.: The doctors fed me on lodlum tea and epecac [...] they then tried agur-forty — if it had been agur-hundred, ’t would n’t have done.
1867		N.O. Picayune 25 Dec. n.p.: Your Honor needn’t say another word; I knock under; this man’s whiskey ain’t Red Eye, it ain’t Chain Lightnin’ either, it’s regular Agur-forty, and there isn’t a man living can stand a glass and keep his senses.
1872		Schele De Vere Americanisms 577: Agur-forty, a curious corruption, showing the almost irrepressible tendency of the uneducated to give some intelligible and suggestive form to terms which they do not comprehend. It is the aqua fortis of medicine.
1944		PADS II 38: Aggie forties [...] Anything very strong; generally used in reference to some-thing to drink.
1947		V. Randolph Ozark Superstitions (1964) 109: In Pineville, Missouri, my old neighbors asked the druggist for ‘a dime’s worth of acker fortis an’ a nickel’s worth of quicksilver,’ by which they meant nitric acid and mercury, to make some kind of itch medicine.
1967–68		in DARE.