plumb - Dictionary definition and meaning for word plumb
plumb--plumber who tests the pipe to see whether it is exactly vertical or not.
On the lines of Mann007: a PLUMBER EXAMINES a pipe CRITICALLY to see if it is VERTICAL and so MEASURES DEPTH of the tank.
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