'cornerstone' originates from Middle English, specifically the words 'corner' and 'stone', where 'corner' meant 'angle' and 'stone' referred to a building block.

'corner' and 'stone' combined in Middle English to form 'cornerstone', eventually becoming the modern English word 'cornerstone'.

Initially, it referred to a literal stone at the corner of a building, but over time it evolved to mean a fundamental principle or element.