“It basically means you’re up to do something for someone. In other words, you’re in for the game or the task at hand. For Doc Holliday, he meant that he was ready to fight the taunting Johnny Ringo.”
2:47 video from “Tombstone” (1993)
It means “pick me or choose me” instead of someone else. Or that you are ready for the struggle or fight.
Thank you. I have learned something today.