Chancy is one of the three delivery men who agree to smuggle dangerous chemicals into Varrock during the Biohazard quest. His name is derived from his gambling addiction, gambling being based upon chance. During Biohazard, you must give him the liquid honey.
If you talk to him on a free-to-play world he will say, "Leave me alone - I'm trying to find my gambling buddies! Whenever you want, come back on a member's world and I'll speak to you then." If you talk to him after you complete Biohazard, you will get a message in the chatbox saying, "Chancy doesn't feel like talking." In Rimmington, you can find Chancy playing with a deck of cards at the camp north of the chemist's house. In Varrock, you can find him sitting at a table in the Dancing Donkey Inn waiting for his gambling buddies. You will find him with Da Vinci and Hops.