Credulous \KREJ-uh-lus\ (adjective) - 1 : Ready or inclined to believe on slight or uncertain evidence. 2 : Based on or proceeding from a disposition to believe too readily.
"Credulous monarchs were easy game for the numerous charlatans and tricksters who toured the courts of Europe trying to dupe them into parting with real gold by means of little more than a promise that they would repay such investments thousandfold." -- Janet Gleeson, 'The Arcanum'
Credulous derives from Latin credulus , "believing easily," from credere , "to believe."