The porn video tag "tranny-suck" refers to a scene or content where a transgender individual, specifically one identified as a woman (usually a trans woman), performs oral sex on another person, typically a man. In this context, the term "tranny" is short for "transsexual" and "suck" indicates that the act being performed is oral sex. This tag is used to categorize content containing such scenes for adult viewers who may be interested in this specific type of material.