Q From Chris Heinrich: Why is a square a socially backwards or overly conservative person?
A We don’t know the reason why the word took on the meaning of somebody who is boringly conventional or old-fashioned. It seems to have been in use first in the jazz world from the 1920s onward, with the first written record said to be in a 1938 jazz catalogue. At first it referred to people who didn’t appreciate jazz; only after the Second World War did it branch out into the wider world with the more general meaning we now know. It’s probable that it’s an appropriation of the figurative sense of “square”, which has been around for many years, of something which is properly arranged and in good order, or which is honest or straightforward (as in “square deal”, or “square shooter”). To move from this idea to a sense of “boringly conventional” is not such a large step, at least from the perspective of speakers who don’t consider themselves to be part of the mainstream.