Describe yourself as wary if you don't quite trust someone or something and want to proceed with caution. Be wary of risky things like wild mushrooms and Internet deals!
You can trace wary through Old English back to Old High German giwar "aware, attentive." If you keep a wary eye on something, you are attentive for signs that it is becoming dangerous. Likewise, if you give someone a wary glance, your face conveys the suspicion and caution you feel. When you are wary of driving alone at night or making promises, you fear something bad might happen if you do these things.