The expression first appeared in Charles Dickens’ well-known classic “David Copperfield”. Barkis is the name of one the characters in the novel. He keeps sending a message through David to Clara Peggotty, the maid of David’s mother. The message that David is to give Clara is “Barkis is willing”. In other words, Barkis is willing to marry Clara. The expression is used nowadays to indicate one’s willingness or readiness to do something.