It should actually be 'carnal' knowledge, not 'cardinal' knowledge, and it's one of those fake acronyms someone invented as a joke origin of a word. I'll leave you to figure out the word (and to look up 'carnal' in a dictionary if you so desire!)
There is a story that the origin of a certain word is the acronym of For Unlawful Carnal Knowledge
or illegal sexual activity. I don't remember the details - perhaps the accused had to wear a sign with that written on it for public shame. It may just be legend, anyway.
Your phrase is a reference to this.
I would just like to point out that this thread began in October 2004, then got picked up again this morning.
As Vanessa, who started the thread, has made only one post, on 4 October 2004, it seems unlikely that she will ever be back to learn from this erudite elucidation of the fanciful formulation of the four-letter.
On grounds of respect for the dead, I propose to re-inter this thread.