Tough-as-nails is an idiom used to describe someone who is extremely resilient and determined. It is derived from the phrase 'tough as IRON, ' which is a reference to the strength and durability of the metal. The crossword solution for this term is 'IRON, ' which is a fitting representation of the phrase's meaning. IRON is known for its strength and resilience, making it an apt metaphor for someone who is tough-as-nails.