Chamaquear (cha-mac-KEHAR) means to use someone's lack of experience to either tease or profit from that person. Its root is the word chamaco, which means child. Children are easier to trick than most adults, hence the reference in chamaquear and in two other expressions: estar chavo and pendejo.
When a person tells you ¡estás chavo! (s-TAZ CHA-vough, lit. you're a kid), that person means that you're inmature and you don't really know the way things work. Pendejo has already been described in detail in this blog.
Note: Chilangos do not think children are dumb and don't get it. The rerefences above are not to the lack of intelligence but the lack of malice, which is thought to be an essential element in personal and professional success.