Noob Tube refers to a grenade launcher in basically every game – especially when the grenade launcher is accessed as a ‘secondary-fire’ of the otherwise close-range shotgun, such as in DOOM, CS1.6 and probably CS:GO, and most of the COD series.
A noob tube, or boob toob, is always derided by the player on the receiving end as requiring no skill because of the explosive ordanance.
‘Noob Tube’ Gaming Slang Term Worldwide Use Trend