The quality or state of being dumb, stupid, or slow-witted; lack of intelligence or understanding.
From 'dummy' or 'dumb' with the suffix '-ness', which creates abstract nouns from adjectives. The word evolved from 'dumb' (unable to speak) to include stupidity more broadly.
The '-ness' suffix is English's most common way to turn adjectives into qualities—kindness, sadness, dumminess. It's so automatic that we can create new ones on the fly and everyone understands!
Complete word intelligence in one call. Free tier — 50 lookups/day.