the.com/dead of winter

means The deepest, coldest, darkest stretch of winter, when the season is at its most unrelenting and furthest from any thaw.

from 'Dead' here doesn't mean lifeless exactly — it's the older sense of 'dead' as the utter stillness or depth of something, the same sense in 'dead of night.' That usage traces back to Old English 'dēad,' and by extension came to mark the flattest, most motionless point of a stretch of time — the hour when night is deepest, or the week when winter has stopped pretending it might ease up.