ודוק or ודו"ק appears extensively in rabbinic literature, particularly in Ashkenazi Halachic writings since the 17th century.
I was once told that it meant that the author thought the proposed statement previously mentioned was unconvincing or a stretch.
I ran into some problems with that translation, and after a while I asked again what it meant, this time being told it meant that the author was asserting the correctness of the previous statement.
This Hebrew-language website gives a kind of convoluted answer, and convoluted answers in languages other than English are not my strong suit. I think it's saying that the expression means that the previous statement requires deep analysis. But it seems to me that it conveys some meaning beyond that.
So what exactly does it mean, and when is it employed?