Square! A shape so definite and final that nothing can change it. There is an air of finality about it. Every side has to match the opposite side and all four sides have to be equal. Its corners are at right angles of 90 degrees each. There is absolutely no room for even the slightest mistake otherwise it would not be a square. So controlled and unbending—so rigid.
The space it holds between the four rigid lines seems trapped. It has nowhere to go. Help! It rushes to each of the four corners seeking escape, but there are no exits out of the square. It excludes everything that falls out of its domain. It just looks so snobbish. As if it is saying, if you are not within these perfect lines and perfect corners you are nothing! Keep out! Maybe that is the origin of the expression “—is so square.”