Slip somebody's memory/mind - Idioms by The Free Dictionary
slip somebody's memory/mind
slip one's mind
[for something that was to be remembered] to be forgotten. I meant to go to the grocery store on the way home, but it slipped my mind. My birthday slipped my mind. I guess I wanted to forget it.
slip somebody’s ˈmemory/ˈmind forget about something or forget to do something: I was supposed to go to the dentist today, but it completely slipped my mind.