A Derogatory term used to compare either; a) Someones intelligence to that of an animal or, b) Someones behaviour and/or character flaws similar to that of an animal.
Guy 1: "These girls are all over you, if you play your cards right you may get a threesome mate!"
Guy 2: "Even if I just play my cards I'll get a threesome, the fucking pigs"
Guy 1: "Farmyard threesome!"