'brat' originates from Old Irish, specifically the word 'bratt', where 'bratt' meant 'cloak' or 'rag'.

'bratt' was used in Scots and northern Middle English to mean 'cloak' or a 'tattered piece of cloth', later extending to mean 'ragged person' and by the early modern period came to be applied to ill-behaved or ragged children, eventually becoming the modern English word 'brat'.

Initially, it meant 'cloak' or 'rag', but over time it evolved into the current common meaning of 'a spoiled or ill-behaved child'.