wouldn't know

I wouldn't know ((anything) about that)

I have no knowledge or familiarity with what you are asking or discussing. Can be used glibly or sarcastically. A: "I absolutely love Barbados. It's one of the most beautiful places in the world!" B: "Hmm, I wouldn't know. I've never been able to afford a trip there." A: "I was wondering if you could help me. I need to fill out a form to have the deadline for my application extended." B: "I'm sorry, I wouldn't know anything about that." A: "Do you know how these drugs got here?" B: "No sir, officer, I wouldn't know about that."
See also: know
Farlex Dictionary of Idioms.

(I) wouldn't know.

There is no way that I would know the answer to that question. John: When will the flight from Miami get in? Jane: Sorry, I wouldn't know. Bob: Are there many fish in the Amazon River? Mary: Gee, I wouldn't know.
See also: know
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
See also:
  • I wouldn't know ((anything) about that)
  • (Is) that everything?
  • Where have you been all my life?
  • this just in
  • (one) will be lucky
  • (one) would be lucky
  • (I) would if I could
  • Who would have thought it?
  • (one) should be so lucky
  • (well,) I never!
References in classic literature
If he was crazy, he wouldn't know, and I wouldn't know his mind either, and I wouldn't be that one jump ahead of him, and he'd get me and mess the whole cage up with my insides."
Michael Brother of Jerry
Besides, the planter insisted that Roxy wouldn't know where she was, at first, and that by the time she found out she would already have been contented.
Puddnhead Wilson
I set down again, a-shaking all over, and got out my pipe for a smoke; for the house was all as still as death now, and so the widow wouldn't know. Well, after a long time I heard the clock away off in the town go boom -- boom -- boom -- twelve licks; and all still again -- stiller than ever.
Adventures of Huckleberry Finn
Jake was going to play deef and dumb here, so the neighbors wouldn't know his voice.
Tom Sawyer Detective
We wouldn't know one from the other, and one of our rival's might slip in."
Tom Swift In The Land Of Wonders
As for the second, he's only half-witted, and if he was better off he wouldn't know it."
A Millionaire of Yesterday
"And even if they did, they wouldn't know who it was and they's keep it mum for me.
The Game
`You wouldn't know the brick house, Sarah,' says Jane.
Rebecca Of Sunnybrook Farm
"Your own mother wouldn't know you like that--much less your sweetheart."
The Kingdom of the Blind
"Sometimes, toward the end, I could see three of him in the ring at once, an' I wouldn't know which to hit an' which to duck ...
The Valley of the Moon
"I wouldn't know how to reach the mainland myself, an' know that none o' the other fellows would, so's you're the only one that knows anything of navigation you're the fellow I'll tie to."
The Beasts of Tarzan
'You wouldn't know him if you saw him then, belike?' said Hugh with a malicious grin.
Barnaby Rudge A Tale Of The Riots Of Eighty
I kept asking myself, 'Now is this the sort of thing Katharine would like?' I always think of you when I'm writing, Katharine, even when it's the sort of thing you wouldn't know about.
Night and Day
"If he did speak, you wouldn't know what to say, but would cry or blush, or let him have his own way, instead of giving a good, decided no."
Little Women
But, sure, you wouldn't know what that is, acushla, nor what it may mean to the likes of me.
The Valley of Fear