The compound adjective "tough-as-nails" is a metaphorical extension built from native English elements. The core word "tough" originates from a Proto-Germanic root meaning tenacious or difficult to break. The comparison "as" links this quality to "nails," specifically the hard metal fasteners, whose name derives from the same Proto-Indo-European root as fingernails and claws, concepts inherently associated with hardness. The idiom crystallized in the 19th-20th centuries, leveraging the literal, proverbial hardness of iron nails to figuratively describe a person's character as exceptionally rugged, unyielding, and capable of withstanding severe pressure, thus creating a vivid and enduring image of mental or physical fortitude.

Structure: tough (strong) + as (comparison) + nails (hard material) - tough: From Old English tōh (strong, tenacious). Functional role: Core adjective describing resilience. - as: From Old English eallswā (just so, even as). Functional role: Preposition establishing a comparative link. - nails: From Old English nægl (fingernail, metal spike). Functional role: Noun used metaphorically to denote extreme hardness and durability.