Charlie: Um, that's not chocolate...
Scott: PEW PEW!!!!!
The Term "Pew pew" is mentioned when an energy weapon such as laser and/or plasma based weapon in movies or games are used by the owner that admittedly confirms lack of confidence of its firepower.
The reason for this term to be used by the possessor of a energy based weapon is mostly due to the seemingly low accuracy and pathetic firepower that cannot intentionally hit its target. This is mostly due to the fact that people who tend to wield energy based weaponry are in one of the following situations:
a) the weapon lacks iron sights or any other means to aim
b) think its cool to be gangsta and shoot from the hip in a pose commonly referred to as "the chimp"
c) have inherent bad vision due to burned retina after too many flashy fire fights.
d) think its cool to see flashy lights and miss intentionally to keep the fight going.
And even in the awkward event of if it actually hitting the target, it sustain next to no damage which then makes the exchange of flashy projectiles and beams continue for an absurdly long time.