To slip your mind is to be forgotten despite meaning to remember. It slipped your mind entirely.
It is a softer admission than forgetting, since the intention was there. It is the standard polite apology when a small thing has been missed.
1phraseslip your mindTo be forgotten despite meaning to remember.
“The appointment will slip your mind unless you write it down.”
slip your mind is a phrase.
Practised in Thinking & Memory