What is the device called that used to be warm a glass of whiskey?

The device you're thinking of is called a whiskey warmer or whiskey heater.

It's typically a small, usually metal, device that can be filled with hot water or heated electrically. The whiskey glass is placed on top of the warmer to gently raise the temperature of the whiskey.

While popular in the past, whiskey warmers are less common today as the practice of warming whiskey has become less popular.