rules of etymology

law of proportion:there is direct proportion between sounds and their meanings,

Are you looking for a dictionary or some form of word source information?

Don't go too far with the onomatopoeia idea. You could end up with all languages using the same words, which simply isn't true.

The Oxford English Dictionary is available as a phone app. I think there's even a free version. The etymologies of the words are included in their definitions.