Literally: four – corner – four – face
Alternately: Square. Four (equal) angles and four (equal sides), just like it says. By extension, a “square” personality: extremely, perhaps even overly, serious. Meticulous; stiff; formal; lacking a proper amount of flexibility and vagueness.
Notes: This really does seem to be setting up a certain “vagueness” as a virtue; several of my sources cite a surfeit of はっきり (“clearly,” “distinctly,” definitively”) as one of the problems of squareness.