The word "credulous" originates from the Latin words "redulus," meaning "inclined to believe" or "trustful."  This Latin term is derived from "credere," which means "to believe" or "to trust". The suffix "-ulus" in "credulus" often carries a diminutive or pejorative sense, suggesting a tendency to believe too readily or naively. Therefore, "credulous" describes someone who is easily persuaded or too trusting. 

According to the Oxford English Dictionary, the word "credulous" was first used in English in the mid-1500s, specifically in 1553.  The Oxford English Dictionary cites a translation by John Brende as its earliest evidence. 

Examples of "credulous" used in a sentence:  
"But then, there's no ear more credulous than the one that yearns to believe."
"We're credulous creatures and easily impressed by things we don't understand."
"The credulous nature of Americans drew only contempt from him." 
"Few people are credulous enough to believe such nonsense." 
"Once the tap shoes come out, even the most credulous viewer has to catch on."
"In that sense, not all the blame can be placed on the most credulous members of the public." 
"Credulous people can often be easy targets for scams." 
"Young children are very credulous with anything that a respected adult says." 
"Since she always tries to find the best in people, Jen can be described as credulous." 
"Kara took the joke seriously because she was too credulous to understand the sarcasm."