'flare-up' originates from the combination of 'flare' and 'up', where 'flare' meant 'to blaze with a sudden burst of flame or light'.

'flare' changed from the Old English word 'flær' and eventually became the modern English word 'flare'.

Initially, it meant 'to blaze suddenly', but over time it evolved into its current meaning of 'a sudden outburst'.