cornerstone (n.) also corner-stone, late 13c., "stone which lies at the corner of two walls and unites them" (often the starting point of a building), hence, figuratively, "that on which anything is founded;" from corner (n.) + stone (n.). The figurative use is biblical (Isaiah xxvii.16, Job xxxviii.6, Ephesians ii.20), rendering Latin lapis angularis.