Something you shout loudly to your teammate in a split screen multiplayer game of some first-person shooter such as Call of Duty whilst you flank the enemy. Flankies also happen to be rather tasty, chocolate-coated strips of meat from a mythical beat known as the chickenfish.
Ex. 1- Dude 1: "Dude! He's coming, he's coming! We're going to die, man!"
Dude 2: "Wait.... wait... okay now! FLANKIES!"
Ex. 2- The starving mountain climbers were greeted at the summit by a divine godess, who presented them with a lifetime supply of flankies which they know had to get down the mountain...