Short for 'Plane Pussy'. The term came about after realizing that in the Pixar movie Planes, the planes must fall in love and reproduce, and to reproduce, the female planes must therefore have a plane pussy.
A term used to describe a member of the LGBTQ+ community that does not fall into the first few letters of the initialism. They are represented by the plus, hence "plussy". Typically meant to be derogatory, but often used by other plussies to make fun of each other.