When the boss's husband shows up at a formal dinner party and proceeds to stuff his pockets full of hors d'oeuvres, no one dares to mention his impropriety above a whisper, especially to the boss.
Impropriety is a violation of a rule of behavior, manners, or etiquette. So while it is fitting and proper to wear a bikini top and a sarong to the beach, to do so at church would be considered an impropriety. Some find a bit of impropriety charming. Somerset Maugham went so far as to say, “Impropriety is the soul of wit.”