Is Plumbness a Real Word?

adj. 1. Exactly vertical. See Synonyms at vertical.

What is Plumness?

: the quality or state of being plumb or vertical.

Is it plum or plumb forgot?

It is a typo. 'Plum' is a tree or its fruit (or meanings derived therefrom). 'Plumb' means the lead weight on the end of a plumb line, used for marking a vertical line when building. Thus the word 'plumb' comes to be used colloquially to mean 'downright'.