Grass-Button is a sex position involving four people of any gender combination. It is when there is a chain of four people having intercourse on their knees outside all at the same time. The origin is the grass is because it is outside and the button part is because a button has four holes and four holes are needed for a successful Grass-Button.
Girl: "Hey I feel like having a Grass-Button, any of your friends wanna come join? it has to be multiples of four though!"