I found a few examples at ALC, but still not sure about the best way to say it.
1 Answer
For example, the most natural way of translating this expression
One day Jennifer tidied up her son's bedroom. Little did she know her son would be so mad when he found that out.
can be rephrased simply as follows:
One day Jennifer tidied up her son's bedroom. She did not know her son would be so mad when he found that out.
Basically it is a fancier way of saying ~ことをまったく知らなかった where ~ is the statement that follows the expression.
I find the most natural way of translating this is to write 彼女は(息子が怒り出す)とは思わなかった, to show that the person was kind of caught off guard by what happened.
little did he know, ...
, there is bound to be a better translation, which does not use this phrase, though, so I am not sure how useful it is.