From Longman Dictionary of Contemporary Englishheaven (only) knowsheaven (only) knowsa)NOT KNOWused to emphasize that you do not know somethingHe won’t tell me what he thinks. Heaven knows why.b)EMPHASIZEused to emphasize what you are sayingSue can’t take a holiday yet, though heaven knows she needs a rest. →heaven
Examples from the Corpus
heaven (only) knows• And heavens knows, once launched it's all too hard to turn back and look again for the missedtrace.• People live together - Peter and Miranda are Bohemian enough, heaven knows.• People wave from the bank and shout our names; heaven knows who they are!• Kenyon must see it through or heaven knows what will become of the station.• What effect it will have on the result, heaven only knows.• If it isn't, heaven knows who will ride to the rescue of the firm.• He starts a conversation and ends up heaven knows where.