Definition
A very cold night.
Origins
An Eskimo saying, used to indicate a low temperature during the night, and during very frigid temperatures, one would require three sled dogs in the igloo to keep oneself warm.
See Also
How People Actually Use It
In the old days of no central heating, a night so cold it took 3 dogs sleeping with you to keep you warm enough.
"Man, last night was real cold, definitely a three dog night."
Source: Urban Dictionary