The tag "bigdaddy51" in a porn video typically refers to a specific porn star or content creator who goes by that name. In this case, the person is likely known for their performance, size, or other distinct characteristics in their adult films and scenes. The number "51" could potentially be a reference to their birthday, height, weight, or another personal detail. As an adult video tag, it is used as a descriptor for videos featuring this particular performer and can help viewers find content related to them. However, the specific meaning may vary depending on the context in which the tag is used.