slip mind

slip (one's) mind

To forget something, I'm sorry I didn't call you back sooner, it totally slipped my mind.
See also: mind, slip

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.
See also: mind, slip