Not predictable; unable to tell what is coming next.
My work schedule is so unpredictable.
"I'd rather go out with someone a little more unpredictable."
Is that what anyone would want in a spouse or a sweetheart? Somebody who is inconsistent, flaky, fickle, erratic, and just unreliable? A bad boy would be just the wrong man to marry if you want a successful, stable marriage.