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

'brat' changed from Middle English word 'brat' (from Old Irish 'bratt') meaning 'cloak' or 'rag' and later was applied to ragged people or children, eventually becoming the modern English 'brat' meaning 'child, often ill-behaved or spoiled'.

Initially, it meant 'cloak' or 'rag', but over time it evolved into its current meaning of 'a badly behaved or spoiled child'.