Question: why is lbw called plumb?
Answer: The term “Plumb” with respect to lbw came from, the building trade, in which there's a thing called a plumb line (usually a lead weight on a cord), which hangs down so they can make sure the walls and doorways are straight. So “plumb lbw” would signify a dead straight lbw (and a nice easy decision for the umpire!).