Plumb means perfectly straight up and down, exactly vertical. When builders check if a wall is plumb, they're making sure it stands straight rather than leaning to one side. A doorframe that isn't plumb may not close properly because the door can swing crooked.

A plumb bob is a pointed weight hanging from a string. Since gravity always pulls straight down, the string shows perfect vertical. Builders still use this simple tool today alongside modern lasers.

You can also use plumb to mean completely or utterly, as in “That story is plumb crazy” or “I'm plumb tired.” This usage sounds old-fashioned and conversational, like something a grandfather might say.

As a verb, to plumb something means to measure its depth, like when sailors plumb the ocean. The word also appears in plumbing, the system of pipes that brings water into buildings. These pipes often run vertically through walls. When plumbers install pipes, they need to make sure the vertical ones are truly plumb so water flows correctly.