A goody with several examples online but no authoritative dictionary entry (and no etymology!):
In the suite occupied by Patricia Van Riis, lobster and champagne were the rule. The patrons of Powder River Rose usually ordered mountain oysters and washed them down with forty-rod. And so on down the list: while with Dolores O’Riely, tortillas and prune brandy from the Imperial Valley…—Nathanael West, A Cool Million, ch. 18
Unless you count Mencken:
Other characteristic Americanisms (a few of them borrowed by the English) are red-eye, corn-juice, eye-opener, forty-rod, squirrel-whiskey, phlegm-cutter, moon-shine, hard-cider, apple-jack and corpse-reviver, and the auxiliary drinking terms…The American Language, ch. 3
Ah, but OED comes through:
1889 FARMER Americanisms, * Forty Rod Lightning, whisky of the most villainous description, so called because humorously warranted to kill at forty rods.
Much snappier than 201-meter lightning. Forty rods are also equivalent to 1 furlong.