A mysterious or shy person who unexpectedly turns out to be the best at something.
Popularly used to describe a man who may not initially catch a lady’s attention but is then unpredictably, completely charming (and, if the lady really hits the jackpot, is fantastic in bed). Due to the element of surprise, a dark horse can often beguile even the most beautiful and intelligent ladies into bed.
‘You got Lizzie into bed? Damn, you’re a dark horse man. We’ve all been trying for months’