French: Maître à penser is a French-language phrase, denoting a teacher whom one chooses, in order to learn not just a set of facts or point of view, but a way of thinking. It translates literally as "master for thinking".
To take a French: maître à penser is therefore close to becoming a disciple. The phrase itself can be used to refer to a type of person - an inspirational genius, for example - who naturally would attract followers interested enough to absorb a whole intellectual approach.
A French: maître à penser is therefore possibly something like a mentor, or guru with a possibly beneficent approach. A negative effect of such a master might be to close down all other intellectual avenues in a student, imposing some schematic or monolithic approach. Such a master receives the French pejorative French: maître-penseur.