leery
Careful and suspicious because something might not be safe.
Leery means cautious or suspicious about something, holding back because you're not quite sure if it's safe or trustworthy. When you're leery of something, you're keeping your guard up, watching carefully before you commit.
You might be leery of a friendly-seeming dog if you once got bitten. A student might feel leery about joining a new club after having a bad experience with the last one. Your parents might be leery of a “great deal” that sounds too good to be true, because experience has taught them that sometimes it is.
The word captures that specific feeling of doubt mixed with wariness. It's not quite fear and not quite certainty that something's wrong. It's more like an inner voice saying, “Wait a minute, let me think about this first.” Someone who's leery isn't necessarily refusing or saying no; they're just being careful, taking time to assess the situation before moving forward.
People often become leery through experience. After your older brother's elaborate April Fools' pranks, you'd naturally be leery when he offers you “totally normal” cookies on April 1st. Being appropriately leery can protect you from mistakes, though being leery of everything can mean missing out on good opportunities.