The answer to your question can be found in the fourth paragraph here.
In an American TV series titled The Good Wife, Lawyer A was introducing one case she was handing over to Lawyer B. Here's the snippet of Lawyer A's words as below, that I copied from the subtitle.
Could you please tell me what slam dunk refers to? I understood slam means very powerful, but I don't find a relevant entry for dunk for this context in my dictionary.Jennifer Lewis. 26 years old.Taught 2nd grade. Accused of killing her ex-huband. Prosecution thought it was a slam dunk 45 years, but the jury came back last week deadlocked.