Gullibility is the quality of believing people too quickly, so you are easy to trick. It is the noun for that softness of belief — not a disease, just a habit of taking a story as true before you check. Writers and teachers use it; in ordinary talk people more often say someone is gullible than talk about their gullibility, which sounds a little formal and a little sharp. Though actually the line with kindness is thin: a warm, open person can also be easy to fool. The word points at the danger, not at having a good heart. It is about scams, tall stories, and “too good to be true” promises — believing without a second look.