The deepest; extending to the greatest depth or lowest point.
From 'deep' + '-most' superlative suffix (also seen in 'topmost', 'foremost'). The '-most' suffix derives from Old English superlative formations.
While we now say 'deepest,' 'deepmost' survives in poetic and nautical traditions—think of the 'deepmost trenches' of the ocean where sunlight never reaches.
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