"Fanny" in the context of a porn video tag is referring to a woman's buttocks or behind. It is often used as a euphemism for this body part, especially in some English-speaking regions and cultures. In adult content, scenes involving fanny play typically involve intimate touching, spanking, or penetration from the rear perspective. Note that this term may not be universally understood and could cause confusion, so it's essential to know your audience while using such terminology.