Very well-behaved, reliable, or of excellent quality.
This simile dates back to the 16th century, when gold was the ultimate standard of value and purity. Gold doesn't tarnish or deteriorate, making it a perfect metaphor for consistent good behavior or quality. The phrase was particularly popular in Victorian times when describing well-behaved children.
Gold's incorruptible nature made it the perfect metaphor for moral goodness - just as gold doesn't rust or decay, someone who is 'good as gold' maintains their positive qualities consistently. The phrase also reflects historical periods when gold was the ultimate measure of worth, making it a compliment that carried serious weight in society.
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