As Good As Gold: Origin and Meaning
The idiom as good as gold has changed a lot from its original meaning because it initially referred to something that was real and genuine. In the past, many people conducted business using credit notes, a document that promised payment within a certain amount of time. Some customers paid on time but others delayed payment or used forged or counterfeited credit notes to avoid paying altogether. Being distrustful, many business owners much preferred being paid in gold and silver, as this form of currency was real, tangible and more importantly, the payment was immediate.
The meaning shifted somewhat to show something that is genuinely good, as the examples below indicate.
– I think that our friend is exaggerating when he says that his dog is a horror, because when we go and visit he is always as good as gold and always does what our friend says. (=truly obedient)
– “How are you feeling, mate?”
– “Good as gold.” (=truly well)