One who deems or judges; possibly a variant or dialectal term related to judgment or assessment.
From 'deem' plus the suffix '-er' (one who does), similar to 'deemer.' This appears to be a variant or rare dialectal formation that parallels standard agent-noun construction.
Deener is so rare it might be a beautiful example of linguistic drift—where one valid word formation (deemer) exists while a synonym (deener) also appears in some dialects, showing how English constantly experiments with its own rules even for the same concept.
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