RTFMs big brother, it stands for Problem Exisists Between Chair and Keyboard. A popular term among office network engineers and IT support departments, it arose due to a need to descretly describe arrogant users (who falsly believe the system or computer is at fault) in thier presence with a suitably techinal and legitimate-sounding term.
"My document is GONE!"
"Don't worry ma'am, its just a PEBCAK. John!, We got a PEBCAK on station 4!"
"Ok, I'm on it"
"Don't worry ma'am, we'll fix it"