From Longman Dictionary of Contemporary Englishgo (right/clean) out of somebody’s mindgo (right/clean) out of somebody’s mind (also slip somebody’s mind)FORGET if something goes out of your mind, you forget it, especially because you are very busy I’m sorry. So much has been happening, it went clean out of my mind. It had completely slipped her mind that Dave still had a key to the house. → mind
Examples from the Corpus
It had ... slipped ... mind that• He had seemed thrown for a moment, as though it had genuinely slipped his mind that he was about to be married. 