gull + -ible

source: [【ONE PIECE】チョッパーのGIF画像｜無料GIF画像検索 GIFMAGAZINE 124328]

easily persuaded to believe something; credulous:

〈人が〉信じやすい, だまされやすい

e.g. an attempt to persuade a gullible public to spend their money.

gullibility | ˌɡələˈbilədē | noun

gullibly | -blē | adverb

early 19th century: from gull2+ -ible.