The compound adjective 'square-shouldered' is a descriptive term formed within Modern English. Its first element, 'square', originates from Latin via Old French, carrying the core meaning of a right-angled, quadrilateral shape. The second element, 'shoulder', is a native Old English word for the body part. Combined with the adjectival suffix '-ed', the term literally means 'having shoulders that are square'. The logic of the word relies on the metaphorical application of a geometric shape ('square') to describe the broad, straight, and angular appearance of a person's shoulders, a usage that became common in physical descriptions from the 19th century onward.

Structure: square (shape) + shoulder (body part) + ed (adjective suffix) - square: From Old French esquarre, based on Latin exquadra (to make square). In this compound, it functions as an adjective describing a shape or angle. - shoulder: From Old English sculdor. In this compound, it functions as a noun denoting the body part. - ed: From Old English -ede, an adjectival suffix meaning 'having' or 'characterized by'.