A male friend who is your right-hand man. He's usually there for you, and is there to support you, and is very good at doing so in social situations. He's always there to offer you a wing.
The term originated from it's usage in aviation, and was then used to refer to a man who would help his friend get his target woman. The wingman would distract the target woman's friend while his friend went in for the target.
"Dutch is my wingman, he's always there for support."
"You were a great wingman last night at that party, my friend!"