Superlative form of 'dead'; the most lifeless, inactive, or lacking in liveliness or excitement.
From Old English 'deað' + superlative suffix '-est.' The superlative form appears in English since at least the 13th century, creating the strongest degree of the deadness quality.
Interestingly, 'deadest' can mean both literally the most deceased (deadest leaves on the plant) and figuratively the most boring (that was the deadest party ever), showing how English metaphorically extends death concepts to describe lacking energy.
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