The quality or state of being amateur; lack of skill, experience, or professional training.
From 'amateurish' (adjective) plus the suffix '-ness' (forming nouns expressing a quality or condition), following standard English word-formation patterns.
There's actually something honorable about amateurishness—the word literally comes from 'amateur' meaning 'lover of,' so it originally just meant doing something for love, not profit, before it became a putdown.
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