slip (one's) mind
To be forgotten, especially of an obligation. I'm sorry I didn't call you back sooner, it totally slipped my mind.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
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.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
slip somebody’s ˈmemory/ˈmindforget about something or forget to do something: I was supposed to go to the dentist today, but it completely slipped my mind.
Farlex Partner Idioms Dictionary © Farlex 2017