'bairnly' originates from Scots, specifically built from the noun 'bairn' and the adjectival/derived suffix '-ly' (from Old English '-lic'), where 'bairn' meant 'child'.

'bairn' changed from Old English 'bearn' and Middle English forms (e.g. 'barn'/'bairn') and became established in Scots as 'bairn'; the adjective 'bairnly' arose by adding the adjectival suffix (compare Old English '-lic' → Middle English '-ly').

Initially the root referred simply to 'child' ('bairn'); the derived adjective originally meant 'of or relating to a child' and has come to be used for 'childlike' or 'suitable for children' in modern usage.