"Ill-divining" refers to a soul that is bad at predicting the future or interpreting omens. In this context, Juliet is lamenting Romeo's departure and the events that have transpired, especially Romeo's killing of Tybalt. She feels that her soul, or her intuition, failed to foresee the tragic turn of events:
* "O, ill-divining soul! Revolted stumbling block! Why should st thou fear to speak?" (Act 3, Scene 5, Lines 50-51)
Juliet feels like her soul was "revolted" or wrong in its predictions, acting like a stumbling block that prevented her from seeing the truth. She criticizes her own intuition for not being more insightful and for allowing her to feel a sense of false security. She is questioning why she didn't anticipate the disastrous outcome of the events.
In essence, Juliet is expressing her deep regret and anguish at the events that have unfolded, and she blames her own intuition, her "ill-divining soul," for not predicting the tragedy.