The quality or state of being brash; reckless boldness combined with harsh or tactless behavior.
From brash plus the suffix '-iness' (an alternative to '-ness' used with adjectives ending in '-y'). The word entered modern English as we needed to discuss personality traits more abstractly.
The '-iness' suffix (used in 'brashiness,' 'happiness,' 'laziness') specifically attaches to adjectives that end in '-y,' showing how English has subtle rules for abstract noun formation.
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