Lengthwise or along the length; at full length; in a prone or prostrate position.
From 'flat' + 'long', an archaic English adverb combining the ideas of being flat and extended along the long axis. More common in older literature and nautical terminology.
You'll find 'flatlong' in old nautical logs describing how something is positioned on a ship—if a sail or cargo is 'flatlong' it runs along the length of the vessel, which is weirdly specific but efficient for giving precise instructions!
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