A chameleon is a middle-sized reptile that can change colors.
They are rather bizarre-looking, because their eyes can point in different directions.
They are famous for their ability to change their color to match their surroundings, hence the nickname, "Chameleon" given to anyone who changes with what's "Popular".
me: I just saw a chameleon!
friend: oh, they are so lame! they think they have a life but they don't
me: no, I mean the reptile