What does an attorney mean when you ask him for an update on your custody case and his reply is " I know it's dirty". The case has not been filed yet and we are in the stage of gathering evidence

Who knows? It's not a legal term. Did you ask him?
Looking back at your story from October, I would guess that he means the entire mess. This question should be merged.

I will agree, when he said that, why did you not just ask him to explain.

But normally the term dirty in divorce means that one or both parties are going to drag many moral issues into court and the fight will be done in a way to harm the reputation of the other person.

So, as noted, if your separation and time with the ex, has a lot of drama, expect that to be told in court. Also some sides even lie or make things sound much worst.

I read your other post, and you do have a mess going and that's what your attorney meant.

https://www.askmehelpdesk.com/family-law/custody-orders-protection-802465.html