sour mash

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A mixture of new mash and mash from a preceding run used to distill certain malt whiskeys.
• noun Whiskey so distilled.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a mash with optimum acidity for yeast fermentation; a mixture of old and new mash; used in distilling some whiskeys
• noun any whiskey distilled from sour mash