http://www.youtube.com/watch?v=W6Wy88tAfWA
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The Urban Dictionary defines as number 2 a reargunner as
An accessory to the more commonly known "wing man," the rear gunner's
job is to provide even more assistance to the lead and the wing men. The
reargunner's job is to hurl insults and other harmful comments about
the physical traits or mental insecurities of anyone and everyone that
tries to approach either of his buddies. The reargunner is a necessity
for serious pick up situations, a job to be entrusted to a true friend
and serious **** talker.