To know the system inside put
To know every detail of it
To know his/her stuff
Very good knowlege
To have a familiar ring to it
To sounds familiar
Name rings a bell
To have a vague memory of someone with this name
Don’t have a clue
Not to know at all
Don’t have the faintest idea
Really don’t know at all
Don’t have the foggiest idea
I absolutely don’t know at all
Can’t for the life of me remember…
I can not remember at all
I’m a bit out of touch
I used to know about but don’t know the latest development
Doesn’t ring any bells with me
I don’t know I’ve ever heard it before
Put two and two together
To conclude from facts I knew
I got (hold of) the wrong end of the stick
Came to the wrong conclusion