1
general::
Don't even think of it. This expression is used as a wish that what was just mentioned will never happen. For example, He's going to give another speech? Perish the thought! This phrase appeared in Handel's oratorio Joshua (1748; text by Thomas Morell): "It never shall be said that our allies in vain implor'd our aid. Perish the thought!" Also see GOD FORBID.
American Heritage Idioms
2
general::
Phrase(s): Perish the thought.
Fig. Do not even consider thinking of such a (negative) thing. • If you should become ill— perish the thought—I’d take care of you. • I’m afraid that we need a new car. Perish the thought.
McGrawhill's American Idioms And Phrasal Verbs