If you are gullible, the joke is on you because you are easily fooled. It is thought that gullible might be derived from the verb gull, meaning "to swallow." This would be a funny coincidence as gullible describes an overly trusting person who tends to swallow the stories he hears whole. GULLIBILITY definition: 1. the quality of being easily deceived or tricked, and too willing to believe everything that…. Learn more. Gullible means easily deceived or duped, naive, or credulous. Find the origin, pronunciation, and translations of gullible, as well as related words and antonyms, in this comprehensive online dictionary. Master the word "GULLIBLE" in English: definitions, translations, synonyms, pronunciations, examples, and grammar insights - all in one complete resource.