How meanings came about

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Hello there, I'm looking for a website to read about how phases and meanings become and what they mean... IE..... as long as a horse's tooth...... I know that means old before your time and comes from a horse getting older and showing more tooth in its mouth. I did read how the saying glory hole came about but for the life of me can't remeber now. There must be a website about that is full of short sayings like this and where they came from etc Plus if you know any please tell me along side with any web addresses you know. Many thanks Anne18