'unflustered' originates from the prefix 'un-' meaning 'not' and the word 'flustered,' which comes from Middle English 'flustren,' meaning 'to be agitated.'

'flustren' transformed into the modern English word 'flustered,' and with the addition of the prefix 'un-,' it became 'unflustered.'

Initially, 'flustered' meant 'to be agitated,' and 'unflustered' has consistently meant 'not agitated or calm.'