branch water

1. Plain water, especially when mixed with a liquor such as whiskey.
2. Chiefly Southern U.S. Water from a stream.

[BRANCH, stream + WATER.]

A term first used in the 1800s referring to pure, clean water from a tiny stream called a "branch". An order for "bourbon and branch" is a nostalgic request for bourbon and water.

The noun has one meaning:

Meaning #1: pure natural water from a stream or brook; often distinguished from soda water