Context #1:

Terrence dials a number on his cell phone. His
friend, Will, answers.

Will: Hello?
Terrence:Will? Where are you?
Will: At home. What are you doing, Terrence?
Terrence: Waiting for you! We agreed to meet in Downtown, remember?
Will: I’m so sorry, Will. It totally slipped my mind. I’m on my way!


Context #2:

Linda and John arrive home to find dinner in
the oven and ​it’s ​burnt.

Linda: John, weren’t you supposed to turn the
oven off before we left?

John: I guess it slipped my mind.

Meaning: to forget. We use this idiom when we have a thought or intention and that thought or intention “slips away” before it becomes an action. 
https://languagesystems.edu/