'lengthily' originates from the adjective 'lengthy', which is derived from the noun 'length', itself coming from Old English 'lengþu', meaning 'length'.

'length' evolved from the Old English 'lengþu' to the modern English 'length', and 'lengthy' was formed by adding the suffix '-y'.

Initially, 'length' referred to the measurement of something from end to end, but 'lengthy' evolved to describe something that is long in duration or extent.