The quality or state of being cubbish; the characteristic of resembling or behaving like a young animal, with playfulness or lack of sophistication.
From cubbish (resembling a cub) + -ness (suffix forming nouns indicating quality or state). The -ness suffix is one of the oldest and most productive in English, tracing back to Old English.
The -ness suffix is so common that we barely notice it—but it's incredibly powerful! You can add it to almost any adjective to turn it into an abstract noun. This is why English speakers intuitively understand words they've never heard before.
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