wir haben es nicht gewußt

Etymology
.

It refers to the stereotypical defense said to have been used by Germans attempting to deflect accusations of not having done enough to stop Nazi crimes against humanity during the Second World War, especially the Holocaust.

Usage notes
When used in the second sense it is likely to backfire, especially when the claim of ignorance is questioned or the reference to war crimes is disproportional to the thing ignorance is claimed about.