A haka is a traditional dance form of the Māori of New Zealand. It is a posture dance with shouted accompaniment, performed by a group.
In recent years, this dance has been adopted by the New Zealand All Blacks, the national rugby team of New Zealand, and perennially the most dominant team in the world, rivaled by Australia and South Africa.
I used to play rugby for Atlanta High Country, Richmond United, Memphis City Rugby Club, and Rhodes College, but retired several years ago due to head and neck injuries.
Oh, and fair warning, if you find yourself at a rugby party, and decide to sing, be sure to get the song right. You don't want to shoot the boot.