A sex act involving four people. Essentially a 69 position taken to the next dimension.
Requirements for the dirty rhombus are that all of the participants must be of roughly the same length from genitals to mouth so that when they are engaged in the act of blowing each other or eating each other out they form a quadrilateral with equal sides.
Billy and I are going to get those two girls involved in a dirty rhombus.
Billy and I just dirty rhombied those two girls.