'bairnie' originates from Scots, specifically formed from the word 'bairn' with the diminutive/affectionate suffix '-ie'.

'bairn' changed from Old English 'bearn' (and Proto-Germanic '*barną') through Middle English forms such as 'bern'/'barn' and became Scots 'bairn'; later the diminutive/affectionate form 'bairnie' developed in Scots dialect.

Initially, it meant 'child' in general; over time it kept that core meaning but also came to be used commonly as an affectionate term meaning 'little one' or 'darling'.