A peculiarity of behavior; an idiosyncrasy: "Every man had his own quirks and twists\u201d ( Harriet Beecher Stowe). An unpredictable or unaccountable act or event; a vagary: a quirk of fate. A sudden sharp turn or twist. An equivocation; a quibble. Architecture A lengthwise groove on a molding between the convex upper part and the soffit.