a small mass of lead or other heavy material, as that suspended by a line and used to measure the depth of water or to ascertain a vertical line. Compare plumb line.
–adjective
2.
true according to a plumb line; perpendicular.
3.
Informal. downright or absolute.
–adverb
4.
in a perpendicular or vertical direction.
5.
exactly, precisely, or directly.
6.
Informal. completely or absolutely: She was plumb mad. You're plumb right.