Pipboy is the electronic data organizer that you use in the Fallout series of RPG games as an inventory, skills and stats screen, quest screen, etc. The Pipboy 2000 is your ultimate guide to the wastes, and key to your character's survival. As for the actual Pip Boy, he is the ever-smiling graphical representation of everything in the Pipboy.
"My pipboy says it's August 15, 2042"
"The pipboy screen shows that I'm low on inventory space."
"Hey, in this picture, he's on fire! ... But he's still smiling."
Used in replacement of "huh?", "say what?", "for real?", etc.
guy1: "hey you whimsical #@$%"