I found this sentence and its translation on tatoeba.org. Is there a better way to translate it into English?
That's a layman's idea.
I looked up 外行, and it does mean amateur or layman, but I've never heard anybody use the phrase "layman's idea" in English. Perhaps a better translation would be as simple as, "That's a dumb idea." Or does the Chinese phrase emphasize that it's a bad idea because the person is inexperienced? If so, perhaps it would be best to translate as, "That's a rookie mistake."