-40 degrees Celsius is equivalent to -40 degrees Fahrenheit. At this temperature, it's extremely cold, regardless of the temperature scale you use. It is the point where the Celsius and Fahrenheit scales intersect, and it represents a common value for very cold temperatures.
To give you some context, at -40 degrees Celsius/Fahrenheit, exposed skin can quickly become frostbitten, and it's important to take precautions to protect yourself from such extreme cold. It's a temperature often associated with frigid climates and severe winter conditions.