"mrthong17" is a slang term in the adult content world, which refers to a specific type of pornographic video or scene. It could potentially mean "Mr. Big Thick Cock #17", but this might not be an official title or numbering system. It can also mean something like "Mr. Big Thong #17", referring to a man wearing a particular type of underwear in the content. Since it's not specific English, there could be multiple interpretations depending on regional usage and context. However, generally this term is used to describe a man with a prominent penis or someone who appreciates them in pornographic content.