The quality or state of being childish; the characteristic behavior, speech, or manner of a child when found in someone older.
From 'bairnish' (adjective) plus '-ness' (the standard English suffix creating abstract nouns from adjectives), fully English morphology applied to Scottish roots.
Bairnishness is a beautiful example of how English word-building works—you can take almost any adjective, add '-ness,' and create an abstract noun describing that quality.
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