'truthful' originates from the word 'truth' combined with the suffix '-ful', where 'truth' meant 'fidelity to an original or standard'.

'truth' changed from Old English word 'trēowþ' and eventually became the modern English word 'truth'.

Initially, it meant 'fidelity to an original or standard', but over time it evolved into its current meaning of 'characterized by telling the truth'.