PS Puru Muru: if it's a man you don't know, you 'don't know him from Adam'.
Thanks for all!
The question arose because an Estonian idiom, literally "I know/understand neither night nor sky about it" has changed in the youth slang into "I know/understand neither night nor hat about it", and it was assumed that the reason for the change was the example of an English similar idiom. My opinion was that there is no such example and it seems that I was right.
I think you're right.
Other alternatives that haven't been mentioned yet:
I don't know squat (or an alternative word that uses the same beginning and ending letters)
I haven't a clue/Not a clue / No clue (this last has a bit of an American tinge to my ear)
That's a closed book to me
(The subject needn't be 'I' in any of these cases.)
None that I am aware of here in the US. Odd that it changed from sky to hat.
"I haven't got a clue" is common. It also has the virtue of being neither pejorative nor profane.
Last edited by Tdol; 19-Sep-2014 at 19:59.