The quality or state of being assish; stubbornness or foolishness.
From 'assish' plus '-ness' (state or quality). Like 'assishly,' this represents multiple suffix layers building a more abstract noun from a concrete animal name.
Notice how '-ness' compounds keep appearing in this word family—English speakers love turning any adjective into an abstract noun, which means we can debate whether something has 'assishness,' 'stupidity,' or 'obstinacy' as if they were measurable properties.
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