plumb - Dictionary definition and meaning for word plumb
plumb--plumber who tests the pipe to see whether it is exactly vertical or not.
When the leg is exactly in the vetical line(plumb) ,then the umpire examines closely to give lbw.
plumb - lumb - lumbar - lumbar bone of the spine keep us straight/vertical
plumb sounds like sanskrit "lamb" which means perpendicular to the line...imagine horizontal line on which u draw lamb(perpendicular)--->vertical line