an expression of... well nothign really, this is used by people who are excited and happy about something that is in most cases unrelated to the topic at hand.
GuyA: What's up?
GuyB: Nothin Much, How bout you?
GuyA: Ask your sister!
GuyC: ZZOMG LAzER PEW PEW PEW!!
GuyB: <GuyA's name here> fuck you, and <GuyC's name here> WTF?!?!?!?!!1
is a word you use when you want to shoot someone of course notfor real, and you can also use it in your instagram bio, be like pew pew, it also means you are shooting a gun on a horse wearing very casual cowboy clothes