if something slips your mind, you forget about it.
I meant to tell her Nigel had phoned, but it completely slipped my mind.
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.
slip your mind
If something slips your mind, you forget it. I was supposed to call her this morning and it slipped my mind.The reason for my visit had obviously slipped his mind.