An interjection. Can be used in nearly any sense, to convey any manner of emotion or simply waste breath and fill up blank space in a conversation. Derives its origin from the 'yarr' used by pirates during the Golden Age of Piracy during the 1500s to 1700s and appropriated from there.
"How are you feeling?"
And so I went up to this dude and he was totally like "Yarr."