A time where it is so cold that the warmest part of your body is affected by it - your balls.
Common during the winter, esp. in Chicago, New York, and Nantucket.
Bro I walked outside yesterday dressed in shorts and a shirt. It was balls cold.
A expression of how cold it is, either used alone, or in a scale. The logical extension of "cold as balls"
Primus: It is balls cold, dude.
Secundus: How may balls?
Primus: Three balls, man. Three balls cold.