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.
Think of it this way. The person who looks like he can never get laid defies the odds and gets any girl he can by being sly,shy and cunning. Although he's not a player, he can really charm the ladies.
Lady: Hello! How are you?
Darkhorse: Good, yea. Haha. Where're you from?
Lady: Oh. Just take me now baby.
did you see that guy! I never thought such a nerdy looking boy could get that chick! That guy is definately a darkhorse.
Basically in reference to, lets say Ohio State in the 2010 NCAA March Madness Bracket challenge. Most people are not expecting Ohio State to win or be in the final four, but it's posibly they could pop up and win, or at the least make a finalfour appearance.
"I think Kansas, Syracuse, Kentucky and Duke will be in the final four, and if there is a darkhorse I think it could be Ohio State"
To act in a sly or secretive manner, rarely letting others know what you're doing, where you're going, or who you're going with.
Being a very shady character, and being quite quiet are essential to perfecting this action.
A good poker-face is obligatory, of course.
synonomus with "sketchy"
when you see a man walking around at midnight, you might say that he is a darkhorse, or that the situation is darkhorse because that is an unusual event
ex. "that guy is a major darkhorse" (that guy is mysterious)
ex. "that game was so darkhorse" (that game was confusing)
ex. "darkhorse!" (exclaiming that something is sketchy)