The term originates from the posture taken by plumbers when doing repair work. It mimics a thickset plumber on his knees inspecting the plumbing system. In this posture, the pants drop low while the shirt rides upwards, exposing the upper part of the bum crack. Of course, this posture can be taken by anyone. But it is stereotyped on plumbers, as it is their common posture as they ply their trade.