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."
Hay Plumb's birth name is Edward Hay-Plumb.
Flora Plumb's birth name is Flora June Plumb.
Gerry Plumb is an artist from Cornwall, England.
Your dads baldhead
Josiah Burr Plumb died in 1888.
Yes, Plumb is a Christian.
Gwen Plumb's birth name is Gwendoline Jean Plumb.
Neely Plumb's birth name is Benjamin Neely Plumb.
Flora Plumb's birth name is Flora June Plumb.
Hay Plumb's birth name is Edward Hay-Plumb.
Use of a plumb bob a level used on plumb rather then horizontal
you spell it plumb
Yes and no. The out-of-plumb wall should be hyphenated while the wall was out of plumb should not.
Robert Plumb was born in 1957.
Ralph Plumb was born in 1816.
Ralph Plumb died in 1903.
Charles Plumb was born in 1864.