"Slenderboy" in the context of a porn video tag refers to a male performer who is slim or thin, typically having a lean and athletic body type. These types of tags are used by adult film enthusiasts to search for specific physical attributes they find appealing or to categorize content based on visual preferences. The term "slenderboy" describes the body build and shape of the male performer, similar to how terms like "thick girl," "curvy woman," or "big boy" describe different female or male performers' bodies in adult films.