Superlative or comparative form meaning 'within' or 'innermost'; an archaic or dialectal word related to 'ben' (Scottish/Northern English for 'inner room').
From ben (Old English/Scottish for 'within, inner') + -most superlative suffix. Common in Scottish English where 'ben' refers to the inner room of a two-room cottage.
In Scottish English, ben means 'mountain' or 'inner room' depending on context, and benmost preserves this older spatial sense—it's the kind of word that vanishes from language but survives in place names like Ben Nevis.
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