The quality or state of being erect; uprightness of posture or the condition of being vertical or rigid.
From 'erect' plus the noun-forming suffix '-ness,' which creates abstract nouns from adjectives. One of the most productive suffixes in English for creating quality-nouns.
The '-ness' suffix is English's favorite tool for turning feelings and qualities into countable things—it's why we can talk about 'erectness' the same way we talk about 'happiness.'
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