'traveller' originates from Old French, specifically the word 'travailler' (and related forms 'travail'), where the stem originally meant 'to toil' or 'work' (ultimately from Latin 'tripalium', an instrument of three stakes).

'travailler' and Old French 'travail' passed into Anglo-Norman and Middle English as verbs such as 'travailen'/'travelen', which shifted in sense from 'to toil' to 'to journey', and the noun form developed as modern English 'travel'/'traveller'.

Initially it meant 'to toil or work', but over time the sense shifted to 'to make a journey' (perhaps via the idea of arduous effort), and the noun came to mean 'one who travels'.