means A four-sided shape with equal sides and right angles, or—by extension—a person seen as conventional, unhip, and dully conformist.

from From Old French 'esquire' and ultimately Latin 'exquadrare,' to make four-cornered (from 'quadra,' a square). For centuries it meant honest and fair—'a square deal,' 'fair and square,' a 'square meal.' The insulting sense is much younger: it bloomed in 1940s American jazz slang, where the in-crowd reportedly mocked the unhip by tracing a square in the air with their fingers, marking the dull and rule-bound. Thus the same shape that once meant honest came to mean tediously straight.