used to indicate that something is thought impossible
as in the phrase
If that works then I'm a Dutchmans uncle.
Of course this does presuppose that you are not a Dutchman's uncle. Why the possibility that one might be is thought so improbable as to become a synonym for impossible is, quite frankly, beyond me. Its not as if the Dutch don't have them.(uncles that is). See Bob's yer Uncle for more related (boom,boom) confusion.