The quality or condition of being deceivable; the degree to which someone is susceptible to being tricked.
From deceivable (capable of being deceived) plus the noun suffix -ness, creating an abstract noun describing the trait or condition of being easily fooled.
This word is so rare that most spell-checkers don't recognize it, but it's technically valid—it's the kind of over-constructed term that shows how English lets you build nearly infinite words from root parts.
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