Strange, unusual, abnormal or bizarre.
Capricious, unpredictable.
Resembling a freak.
Not normal; odd, unusual, surprising, out of the ordinary.
Outside of one's current relationship; unfamiliar.
Having the quantum mechanical property of strangeness.
Not belonging to one.
Unfamiliar, not yet part of one's experience.
Of an attractor: having a fractal structure.
Sex outside of one's current relationship.