An interjection used to signal the presence of a good looking female.
Stems from the sound a robot makes after seeing a hot fem-bot. It is caused by his insides disintegrating (from the uber-sexiness, of course). The pew sound can be imagined as a nut/bolt hitting the inside of his outer casing, similar to the sound made by a ricocheting bullet.
3. Did you see that girl, Angelica's face? Dude. It is so pew pew.
4. (this is how i use pew pew LOL)
Saying something like "heyy girl. how you doin?"
and then winking. and then licking your thumb in a seducive yet sarcastic and hilarious way.
Its not serious flirting. Its just a great way to make a girl either slap the hell out of you (she must think it was serious.. O_o) or make her laugh like hell.