Superlative form of 'ducky'; most ducky, most satisfactory, or most resembling a duck.
From 'ducky' plus the superlative suffix '-est.' This follows standard English formation patterns for expressing the highest degree of a quality.
In the 1920s-40s, British slang used 'the duckiest' to describe something absolutely perfect—it's slang that sounds charming to modern ears but represented genuine enthusiasm at the time.
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