In a clear, obvious manner; without decoration or complexity.
From Latin 'planus' meaning flat or level, through Old French 'plain'. The sense of 'clear' or 'obvious' developed from the idea of something being on a level surface, easily seen.
The word 'plainly' connects physical flatness to mental clarity - when something is 'plain to see,' we're unconsciously using ancient metaphors that link level ground with transparent understanding.
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