In a situation where two guys encounter two (or more) ladies, and one guy would like to strike up a private conversation with one of the ladies, the other guy is designated the wingman. The wingman's duty is to distract her friends, so as to increase the chances of a
favorable outcome for the other guy (
digits,
hookup, etc.).
If the other lady (or ladies) are less than attractive, the wingman will be said to have dived on a grenade. On a successful score by the other guy, the wingman's actions will not go forgotten.