three-dog night
old-fashioned A bitterly cold night (i.e., so cold that one would need their dogs in bed with them to stay warm). Temperatures fell well below freezing, a true three-dog night.
three dog night
A very cold night. Back in pre-central heating days, on nights when a thick quilt wasn't available or didn't provide enough heat, rural dwellers might take a dog into bed to provide additional warmth. Chilly weather called for one dog, even colder for two, but when it was cold enough to freeze a brass monkey, you'd make room for three dogs.