A phrase invoking God's protection to keep something from happening. Sometimes used sarcastically or hyperbolically. God forbid I get another ticket on my parents' car. I'll be grounded for a month! God forbid that an R-rated film should have anything offensive in it!
A phrase used to invoke (at least figuratively) a higher power to prevent something that one believes would be tragic if it were to happen. Often used sarcastically. If, heaven forbid, something were to happen to you, you would want to know that your family would be taken care of. Well, heaven forbid people in power actually listen to their constituents! Heaven forbid that poor family has to endure another tragedy.
Also, heaven forbid. May God prevent something from happening or being the case. For example, God forbid that they actually encounter a bear, or Heaven forbid that the tornado pulls off the roof. This term, in which heaven also stands for "God," does not necessarily imply a belief in God's direct intervention but merely expresses a strong wish. [c. 1225] For a synonym, see perish the thought.