dead of night; middle of the night
Emphatic form of 夜中, emphasizing the deep, quiet hours of the night. Often used in literary or dramatic contexts.
See also: 夜中
I was startled when the phone rang in the dead of night.
The park in the middle of the night was quiet with no one around.
Reduplication of 夜 (よる, 'night') + 夜中 (よなか, 'middle of the night') for emphasis. The exact historical derivation is uncertain, but the form is used to intensify the meaning of 夜中.