From Longman Dictionary of Contemporary EnglishGod/Heaven forbidGod/Heaven forbidspokenHOPE used to emphasize that you hope that something will not happen ‘Supposing I had an accident.’ ‘God forbid!’ → forbid
Examples from the Corpus
God/Heaven forbid• Or, heaven forbid, ridicule him?• My guess is this: Heaven forbid that you have a cancer right now.• And heaven forbid that you should be served by some one in a chain coffee house with a regional accent.• God forbid you should have an accident. 