What does plumb?

Answer 1

In a vertical or perpendicular line. e.g. "The wall is plumb."

This is the most common meaning and alludes to the method of establishing vertical by using a plumb bob - a weight suspended from a long line which hangs straight down - with the resulting line between the top of the line and the plumb bob being a reference for the edge of something that is supposed to be perfectly vertical. If it is further away from the line at the top or bottom of the line than it is at the other end, it is not quite "plumb".

There are, of course, several more informal uses for the term:

(Informal definition) Directly; squarely: e.g. "It fell plumb in the middle."

(sometimes also spelled as "plum" with the informal definition...) Utterly; completely: e.g "I'm plumb worn out."

informal chiefly US (intensifier): e.g. "That's just plumb stupid."