the.com/dead of night

means The darkest, stillest, most silent part of the night, typically the small hours when nearly everyone is asleep.

from Built from "dead" in its old sense of complete stillness or total cessation — the same "dead" you hear in "dead calm," "dead silence," and "dead center," meaning absolute and without motion. English has paired "dead" with the depths of a thing for centuries (the "dead of winter" works the same way), so the "dead of night" is simply night at its deadest: no light, no sound, no stirring. The phrase has been in use since at least the 1500s.