Rudeness (also called effrontery) is a display of actual or perceived disrespect by not complying with the social norms or etiquette expected within a relationship, social group, or culture. Social norms are established as the essential guidelines of normally accepted behavior within a given context, and individuals often establish personal boundaries to meet their own needs and desires within smaller settings, such as friendships. To be unwilling to align one's behavior with these norms known to the general population of what is socially acceptable is to be rude. These norms may resemble a sort of "unspoken law", with social repercussions or rewards for violators or advocates, respectively.

Rudeness, "constituted by deviation from whatever counts as politic in a given social context, is inherently confrontational and disruptive to social equilibrium". Forms of rudeness include behaving in ways that are inconsiderate, insensitive, deliberately offensive, impolite, obscene, or that violate taboos. In some cases, an act of rudeness can go so far as to be a crime (e.g., the crime of hate speech). In many cultures, joking can create a context where rudeness is temporarily excused or indulged for the purpose of making people laugh.

The concept of rudeness is somewhat subjective (i.e., different people may have different views) and contextual (i.e., it depends on the setting). Some actions, such as wearing shoes indoors, using vulgar language, wearing headphones, or addressing elders by their first name, may be deemed rude in one setting and perfectly acceptable or even expected in others.

Both manners and morality deal with whether a thing is morally good or bad, but at different levels. Unlike morality, which, for example, condemns murder as a violation of a person, manners primarily concerns itself with violations of human dignity, rather than the person's health or property. Rude behaviour is a violation of human dignity or of the respect due to others.

The moral basis for opposing rudeness is that people ought to give some consideration to other people's feelings, and thus avoid causing them needless or undue distress.

The specific actions that are considered polite or rude vary dramatically by place, time, and context. Differences in social role, gender, social class, religion, and cultural identity may all affect the appropriateness of a given behaviour. Consequently, a behaviour that is considered perfectly acceptable by one group of people may be considered clearly rude by another.

For example, in medieval and Renaissance Europe, it was rude to indicate that a man wearing a mask in public could be recognized. Instead, polite behaviour demanded that the masked person be treated as a completely unknown person and that no one ever attribute the masked person's actions to the individual who performed them. By contrast, in the modern era, greeting a friend by name while he is wearing a mask, or talking to them later about their costume or activities, is not generally regarded as rude.

Cultural differences also appear over time. In the mid-20th century, the meaning of eye rolling changed from its older signal of lust and passion to expressing contempt.