In a manner showing a tendency to believe things too easily; in a way that's easy to trick.
From 'gullible' plus the adverb-forming suffix '-ly.' Traces back through 'gull' (to deceive) to possibly the gull bird, famous for being easy to catch.
The shift from noun (gull the bird) to verb (to gull someone) to adjective (gullible) to adverb (gullibly) shows how one word can journey through all parts of speech!
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