Which term describes behavior that is not in keeping with accepted standards of polite society?

The term that best describes behavior that is not in keeping with accepted standards of polite society is "improper." This word specifically indicates actions that go against societal norms or expectations regarding conduct, making it an apt choice for this context. It captures the essence of behaving in a way deemed socially unacceptable or offensive.

While "untoward" can imply something inappropriate or unexpected, it is often used in contexts that refer to unforeseen difficulties or adverse circumstances, rather than directly addressing social etiquette. "Peculiar" typically refers to something being strange or unusual, which does not necessarily align with social impropriety. "Uproarious," on the other hand, describes something noisy or humorous, which again does not convey the sense of violating social norms.

Thus, "improper" stands out as the clear choice for describing behavior misaligned with polite societal standards.