A: How do you like my playing, Mr. White?

    B: I have never heard an amateur who can play the piano so well. You are really a dark horse.

    Note: This idiom refers to someone who emerges to prominence, being previously little known. It was originally horse racing parlance. A dark horse was one that wasn’t known to the punters and was difficult to place odds on. The figurative use later spread to other fields and has come to apply to anyone who comes under scrutiny but is previously little known. Benjamin Disraeli provides the earliest known reference to the phrase in “The Young Duke,” 1831: “A dark horse, which had never been thought of ... rushed past the grand stand in sweeping triumph.” The figurative use seems first to have been taken up when referring to the candidates for academic preferment.