I would like to know how the phrase "as good as one's word".
I figured out the meaning of that phrase, but I can't understand how or why it can be made. The word "word" means a promise in this case, but what is the meaning of "as good as"?
Please tell me if you know the origin or the structure of "as good as one's word".
If possible, I would appreciate if you could give me the citation or reference about that phrase.