Designed or constructed in such a way as to prevent or resist harm, damage, or negative effects.
From harm plus -proof (a suffix meaning 'resistant to' or 'protected from'), following the pattern of other -proof compounds like waterproof, soundproof, and weatherproof.
The -proof suffix is a brilliant English creation because it lets speakers instantly create adjectives meaning 'protected from anything'—so we get waterproof, childproof, foolproof all from the same suffix, and new ones are invented constantly to describe products.
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