When you're at a fancy dinner party, if you burp after you eat, use your fingers to spread butter on your bread, and hang spoons from your nose, people will probably say you are uncouth, meaning vulgar and ill-mannered.
The adjective uncouth comes from Old English and it meant "unfamiliar or not well known." As the meaning developed, the word came to mean "rude, vulgar, or lacking refinement." Interestingly, the word uncouth came first and its antonym, couth, was developed to describe someone who is cultured, polished, and sophisticated. Although couth gets an entry in the dictionary, you will still hear the word uncouth used far more often.