Resistant to corrosion, oxidation, or damage from copper; treated to prevent interaction with copper or copper compounds.
Compound of copper and proof (meaning resistant or protected against), following the pattern of waterproof, fireproof, and similar protective adjectives in English.
Making something copperproof is actually a specialized industrial challenge—certain plastics and rubbers react chemically with copper, so engineers have to carefully select materials when building water systems or electrical components that contact copper!
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