Wikipedia
Straight whiskey (or straight whisky), as defined in United States law, is whiskey created by distilling a fermented ( malted or unmalted) cereal grain mash to create a spirit not exceeding 80% alcohol content by volume (abv) and then aging the spirit for at least two years at an abv concentration not exceeding 62.5% at the start of the aging process.
Mixing whiskey from different barrels (and sometimes from different distilleries, although only from within the same state), filtering, and dilution with water (while retaining at least a 40% abv concentration) are the only allowed modifications for straight whiskey prior to its bottling. American straight whiskey must be aged in charred new oak barrels and must be put into the barrels for aging at a concentration not exceeding 62.5% abv. When bottled, only water to reduce proof may be added. This is the definition established for production of American whiskey for consumption within the United States as per the U.S federal Standards of Identity for Distilled Spirits. (The regulations do not necessarily apply to American whiskey made for export.)
Alternatively, straight can also refer to a way of requesting and/or serving a drink of whiskey—pouring the spirit without any water, cordial, or other mixer. Thus, in bartending terminology, straight is typically synonymous with neat.
Usage examples of "straight whiskey".
He stepped forward eagerly, as if anticipating excitement at the next tree, and he had won the respect of his fellow travelers by preaching a simple Christianity characterized by God's abiding love for his children, and the respect of the muleteers by nipping straight whiskey when the nights were cold.
At times, especially on Saturday nights, he came home, still full of straight whiskey and the sound of his own ranting.
The antics of those friendly fools might make him forget Daddy sprawled in the chair, pouring straight whiskey on top of his pain.
He had a Waterford glass that looked full of straight whiskey in one hand.
The waiter brought him straight whiskey and, pretense abandoned, stood by while Hsi drank it.