'lengthiness' originates from the word 'lengthy', which is derived from 'length', itself coming from Old English 'lengþu', meaning 'length'.

'length' changed from the Old English word 'lengþu' and eventually became the modern English word 'length'.

Initially, it meant 'the measure of something from end to end', but over time it evolved into its current meaning of 'the quality of being long or extended'.