Definition from Wiktionary, the free dictionary
- (figurative or humorous) superlative form of : most dead; most dead.
- 1848, Charlotte Bronte, Jane Eyre 
- What crime was this, that lived incarnate in this sequestered mansion, and could neither be expelled nor subdued by the owner? -- what mystery, that broke out now in fire and now in blood, at the deadest hours of night?
- 1915, Kenneth Grahame, The Golden Age 
- Here Rosa fell flat on her back in the deadest of faints. Her limbs were rigid, her eyes glassy; what had Jerry been doing? It must have been something very bad, for her to take on like that.
- 1848, Charlotte Bronte, Jane Eyre