Since the meaning of "opportune" is "favorable" or "well-timed," it's easy to guess that the meaning of inopportune means something that is ill-timed or inappropriate. That little prefix in- means "not," and it totally turns the meaning of a word around.
Think of the Latin word opportunus, which means "fitting" or "opportune." When you add in- in front of it, or any word, you give the word the opposite meaning. That's the history of the word inopportune, which has been used since the early 16th century to describe something that is "not fitting." For example, "She opened the door at an inopportune moment and saw him fire the gun." Bad timing? You bet. And very inopportune.