While playing a multiplayer video game (most specifically Halo, however it can be applied to almost any game) one player is used as a decoy to draw in the enemy team and while they are distracted with killing the decoy other team mates come from behind the enemy team and completely eradicate them. One of the most infamous cases occured in late 2005 on Halo 2. In a close team slayer game one SteelAssassn plaid the decoy on Lockout drawing in an unsuspecting enemy team that was then ambushed by ArcAngel316 (Now ArcAngel613), Holycrapp, and Assasin003 (Now Assassin388) thus taking out the enemy and winning the game by a single point.
The "Willie" portion of the name originates from "Willie" Nelson the famous country singer who smokes marijuana. This is because much like a person who has smoked some mary-jane the enemy team is happy that they have found a lone person to take out for an easy kill.
The "Nixon" portion of the name orginates from President Richard "Nixon". This is because the whole thing is a lie much like Watergate. The lone player is not an easy kill, but a trap.
"We straight Willie Nixon'ed em."
"Set-up the Willie!"
"Don't go in there it's a Willie Nixon!"
"There's no escaping a good Nixon."
"Pull them into the Willie."