-When you and your buddy go out to the bar/a party. And let's say you meet a hot chick, but there's a catch she's got an annoying or ugly friend. It's your buddy's job "The Wingman" to keep the "other chick" occupied so she won't want to leave. No one wants to talk to her, and she's jealous that her "hot friend" is getting attention. While your Wingman is keeping her occupied and preventing any "cockblocking" from happening you are free to make your move(s). You can even use your Wingman as an excuse for her to leave with you. "Look, your friend is having a great time. My buddy and her seem to be hitting it off real well. Come on, let's go back to your place."
As with the pleasure comes the pain, you have to take the Wingman position the next night so your buddy can be the Gunner.
On rare occassion(s) the next morning you will not know who was the Wingman, and who was the Gunner. Because they both looked ugly/annoying at the beginning of the night, but by the end of the night they were both hot!
"Mike took the Wingman position with honor that night."
"Yea, we weren't sure who was the Wingman on that morning."