The cred in credulous is from Latin credere, meaning “to believe” or “to trust.” Credulous describes people who would be wise to be a bit more skeptical, or things that ought to be approached with some skepticism. The word has a useful opposite in the term incredulous, which often describes something that shows or suggests one’s lack of belief (“listening with an incredulous smile”), or someone who cannot or will not believe something, as in “an outrageous statement that left them incredulous.” (You’ll do well not to confuse incredulous with incredible.)