Other ways of saying forget
have no recollection of something
to be completely unable to remember something, so that you think that perhaps it never happened: I have absolutely no recollection of ever seeing this man.
slip your mind
if something slips your mind, you forget it because you are busy doing other things: I’m sorry I didn’t phone, I was working and it slipped my mind.
be on the tip of your tongue
used for saying that you cannot remember a name or fact that you know, but that you think you will remember it soon: What was his name again? It’s on the tip of my tongue.
my mind’s gone blank
used for saying that you cannot remember something, especially when someone has asked you a question