the opposite of 'noob tube', a term used in Call of Duty 4 where one shoots a grenade with the grenade launcher attachment at an enemy who is out in the open and could have easily been killed with a few bullets.
pro pipe is when the grenade launcher attachment is used tactically, ie. in the situation where when an enemy is wounded but escapes to cover behind a wall or around a corner and other enemies are advancing, so the grenade launcher is used in an intelligent way where no other option was available.
player: "yeah, so xXDARK_SN1PERXx took a faceful of bullets from my M16 and managed to escape around a corner, and when I saw his teammates coming up from behind him, I used my grenade launcher to kill him with the splash damage from the explosion."
player's buddy: "nice use of the pro pipe! but, you're a fucking noob for using the M16."