[I]t is well understood that the right of free speech is not absolute at all times and under all circumstances. There are certain well defined and narrowly limited classes of speech, the prevention and punishment of which have never been thought to raise any Constitutional problem. These include the lewd and obscene, the profane, the libelous, and the insulting or "fighting" words -- those which, by their very utterance, inflict injury or tend to incite an immediate breach of the peace. It has been well observed that such utterances are no essential part of any exposition of ideas, and are of such slight social value as a step to truth that any benefit that may be derived from them is clearly outweighed by the social interest in order and morality.
Chaplinsky v New Hampshire, 315 U.S. 568 (1942).