Poindexter

My students are reading The Watsons Go to Birmingham. In it one of the characters is referred to as Poindexter by other students...because he likes to read a lot. As I understand it, it's used to refer to a "brainiac" (as one of my students described it) or a very smart person... maybe a know-it-all?? Where does this term originate and does anyone have a more specific definition or description of it's meaning? Thanks for your help.