Pewpew is a playful way to refer to DPS -- especially ranged damage -- and its associated classes. Usually it is a teasing oversimplification of what DPSers are expected to do in a group: hit things quickly and repeatedly. It is also referenced in the expression less QQ, more pewpew, admonishing players to stop delaying play with discussion about nerfs, damage meters, or general arguing and simply just try harder at the task at hand.
More pejoratively, it is used by healers and tanks who argue not only is every class is able to DPS to some degree, but some classes do it exclusively regardless of spec or the potential for alternate roles, making them ubiquitous, replaceable, and disposable in groups. This contrasts with the danger an underperforming tank or healer creates for the entire party. Thusly, such players are only interested in the damage ultimately performed by DPSers rather than the associated class, spec, or twink minutiae.