PEBCAK is an acronym for "Problem Exists Between Keyboard And Chair".

Used by support people, particularly at call centers and help desks. A useful term for demeaning the incompetent user without actually saying it to their face.

The absolute point o of origin is difficult to nail down. The acronym appears on word-detective in 1998, and given the context, sounds like it was already a commonly-known in-joke among customer support reps of the time.