Which term describes humor that is humorously vulgar?

The term that describes humor that is humorously vulgar is "ribald." This word specifically pertains to humor that is characterized by coarse, irreverent, or indecent themes and is often sexually suggestive in nature. It captures the essence of humor that pushes boundaries and delights in the provocative or the naughty, making it distinct from other forms of humor.

In contrast, the other terms do not carry the same connotation of vulgarity. "Piquant" refers to something that is interesting and attractive, typically used to describe flavors or remarks that are stimulating or intriguing, but not necessarily vulgar. "Witty" describes humor that is clever and quick, emphasizing intelligence rather than ribaldry. "Whimsical" denotes a playful, fanciful quality that brings a light-hearted or capricious tone without the vulgar elements associated with ribald humor. Hence, "ribald" is the most appropriate choice to capture the essence of vulgar humor.