Which adjective describes behavior that is excessively irreverent or mocking?

The adjective "scurrilous" accurately describes behavior that is excessively irreverent or mocking, particularly when it involves using vulgar or coarse language to express contempt or ridicule. This term is often associated with actions or speech that aim to insult or belittle someone else, making it fitting for situations where irreverence is taken to an extreme.

In contrast, the other choices, while relevant in different contexts, do not convey the same connotation of excessive irreverence. "Sneering" implies a contemptuous or mocking expression but lacks the broader implication of vulgarity that "scurrilous" carries. "Respectful" directly opposes the concept of irreverence, suggesting deference and honor towards others. "Humble" refers to a modest or low view of one's importance, which also stands in stark contrast to the mocking attitude embodied in "scurrilous." Thus, "scurrilous" is the most precise term for behavior that crosses the line into excessive mockery or irreverence.