Designed to resist or prevent infestation by insects or bugs; protected against insect damage or intrusion.
From 'bug' plus 'proof' (protected against), following the productive English pattern of combining 'proof' with nouns to indicate resistance (like waterproof, dustproof, soundproof).
The word 'proof' in English originally meant 'tested' (as in 'tested alcohol'), but now it means 'impervious to'—language evolved this meaning through phrases like 'tested against bugs' becoming 'bug-proof.'
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