: The ice serves two functions—it chills the spirit and, as it melts, provides a slight dilution that can "open up" the flavors of a whiskey or rum, making it smoother to drink.

: It stands in contrast to ordering a drink "neat" (poured straight from the bottle at room temperature) or "up" (chilled with ice but strained into a glass without the ice). 2. The Idiom of Instability

Metaphorically, if something is "on the rocks," it is in a state of severe difficulty and likely to fail soon.

In the world of spirits, ordering a drink "on the rocks" means it is served over ice cubes.