I don't think the meaning of this expression has been made clear enough to get a proper translation yet. It is used in two very different ways. The first is when you mention something terrible that might happen in the future. You say "heaven forbid" or "God forbid" before you mention the terrible thing. This means you don't want the horrible thing to happen; you're just raising the subject. "You should have life insurance. What if, heaven forbid, you got hit by a bus and died tomorrow? Who would take care of your wife and kids?" In this case, many people think that saying "heaven forbid" or "God forbid" will actually prevent the bad thing from happening. The second use of "heaven forbid" is very, very sarcastic. We use it to describe the attitude of someone who is being unreasonable, unfair, or selfish. For example, a wife may say to her husband, "I do all the cooking and cleaning in this house. Heaven forbid you ever wash a dish or boil an egg!" She is implying that, in the husband's mind, washing one dish would be a terrible thing that he is desperate to avoid -- as terrible as being struck and killed by a bus.

Unfortunately, an English expression does not always have a corresponding Chinese expression. As for the example above, it means in Chinese: 如果他故伎重演——苍天保佑，不要发生这样的事情（但愿不要发生这样的事情），他自己都知道该怎么办了。 第二句：亡羊补牢当然是对的，但对于毒品而言，更重要的是要防患未然。

My mother said "heaven forbid" quite often. I believe it is primara christian saying. heaven forbid: I hope it does not happen also used to suggest that others oppose the reasonable thing being mentioned I grabbed that from http://idioms.thefreedictionary.com/heaven+forbid

天意不允,有违天意.严重一点可以说:天理不容.

Pardon chekezoe? I very very rarely pick a best answer, I know what he was doing, I have no problem with that.