'bairnish' originates from Scots English, specifically from the word 'bairn' + the adjectival suffix '-ish', where 'bairn' meant 'child' and '-ish' formed adjectives meaning 'like' or 'pertaining to'.

'bairn' comes from Old English 'bearn' meaning 'child'; in Scots it became 'bairn', and the modern adjective 'bairnish' developed by adding the Middle English/Old English-derived suffix '-ish'.

Initially it referred simply to things 'of or relating to a child' (from 'bairn'), but over time it came to be used both affectionately for 'childlike' and pejoratively for 'childish' or 'immature'.