Furthest back or toward the rear; the most rearward or final position.
From Old English 'æfter' (after, rear) + superlative suffix '-most' (from 'most'). Common in nautical terminology since Old English.
Naval architects use 'aftermost' to describe the very rear of a ship, and it's why the captain's quarters were traditionally located there—it was the most secure and prestigious position aboard.
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