In a way that is capable of being deceived or in a manner suggesting susceptibility to deception; in a deceivable manner.
From deceivable (capable of being deceived) plus the adverb suffix -ly, creating an adverb that modifies verbs related to being tricked or fooled.
This adverb is so uncommon that you almost never encounter it in modern writing—it's another example of how English allows theoretically infinite word combinations even if most speakers never use them.
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