If we were to divide the world between systematic thinkers and rappers, accountants and poets, mathematicians and statisticians, then I would be a rapper in love with mathematician. I believe in Socrates and have a strong natural dislike for Aristotle. One was a hero, the other a librarian. The schoolboy in me prefers the former.
# Historical
In referring to a person, the word originally meant someone who was honest, traditional and loyal. An agreement that is equitable on all sides is a "square deal". During the rise of jazz music, the term transformed from a compliment to an insult.
# Negative connotation
The negative connotation of the term has a surprisingly long history. In the parlance of jazz, a square was a person who failed to appreciate the medium, or, more broadly, someone who was out of date or out of touch, hence the saying "be there or be square" - wikipedia 
> The phrase comes from the square representing a four-beat rhythm as shown by a conductor's hands.
In the counterculture movements that started in the 1940s and took momentum in the 1960s a "square" referred to someone who clung to repressive, traditional, stereotypical, one-sided, or "in the box" ways of thinking. The term was used by hipsters in the 1940s, beatniks in the 1950s, hippies in the 1960s, yippies in the 1970s.
# Original Meaning
The original positive meaning of "square" was first attested in the 1560s, a sense preserved in the phrases:
> fair and square > a square deal > squaring up - as in upstanding (to an antagonist).
As a symbol of rectitude, the square, or set-square, is one of the principal allegorical symbols in Freemasonry.