'bairnishness' originates from Scots and Old English roots, specifically from the Scots word 'bairn' (from Old English 'bearn'), where 'bearn' meant 'child', combined with the suffix '-ish' meaning 'having the quality of' and the nominalizing suffix '-ness' meaning 'state or condition'.

'bairnishness' developed from the Scots/Old English root 'bairn' (Old English 'bearn'), which produced the adjective 'bairnish' ('childlike') in Scots and northern English, and was later nominalized as 'bairnishness' to denote the state or quality of being childlike.

Initially it referred simply to something 'pertaining to a child' (childlike), but over time it has been used to denote the broader idea of 'the quality of being childlike'—including both innocent playfulness and, sometimes, childishness.