"Tranny-deepthroat" is a porn video tag that combines two different categories within the adult entertainment industry.
"Tranny" typically refers to transsexual or transgender performers, often meaning male-to-female (MTF) individuals. This term has evolved over time and can have different meanings depending on context and community. However, in this case, it signifies that the video may feature a performer who identifies as trans or transgender.
"Deepthroat," on the other hand, is a specific sexual act depicted in the video. It is an oral sex technique where the performer takes their partner's penis deep into their throat, often to the point where it appears that they are gagging. This tag suggests that this particular act will be performed by the transgender/transsexual performer in the scene.
So, the tag "tranny-deepthroat" implies a porn video featuring a trans or transgender performer who performs oral sex (specifically deepthroating) on their partner.