‘She’s a dark horse,’ he said. ‘She knows just as much about climbing mountains as you or I. In fact, she was ahead of me the whole time, and I lost her.’

As she pulled the door closed behind her, she heard the nurse say, “Well! You’re a dark horse, I must say! Do you know that extraordinary-looking girl?”

“Well!” Genevieve laughs – the kind of bright, trilling laugh you give when you’re really quite annoyed about something. “Ed, you are a dark horse! I had no idea you had a girlfriend!”

Dark horse

Meaning

Noun