not have a clue/have no clue
- "Who invented algebra?" "I don't have a clue."
- Don't ask your father which key to press - he hasn't got a clue about computers.
not have the faintest idea
- "Is she going to stay?" "I haven't the faintest idea."
- I don't have the faintest idea what you're talking about!
not have the foggiest (idea)
- I didn't have the foggiest idea what he was talking about.
can't for the life of me remember
- I can't for the life of me remember her name.
be out of touch
- I didn't see any news all the time I was on holiday, so I'm completely out of touch.
doesn't ring any bells
- No, I'm sorry, that description doesn't ring any bells with me.