"Top-tranny" is a term used in the adult entertainment industry, particularly in transgender pornography. The term "top" typically refers to the active partner in a sexual act, often associated with penetrative roles such as inserting a penis into another person. In this context, the term "tranny" is a slang term for a transgender person.
So, "top-tranny" describes a transgender performer who takes on the active, penetrative role in sexual acts during a porn video. These performers may have a penis and often identify as male but are biologically female. They perform anal or oral sex with other performers, typically males or females, depending on their preference and the scene's content