In a manner that is somewhat dead, lifeless, or lacking vitality.
From 'deadish' (adjective) plus the adverb suffix '-ly' (from Old English '-lice' meaning 'in the manner of'). This double suffix construction shows how English builds nuance through layering.
Notice how English lets you stack suffixes—'dead' to 'deadish' to 'deadishly'—each one softening or modifying the original meaning, like turning up a dimmer switch on intensity.
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