We don't have synonyms for devoir yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are devour, devoid and devor.
"1885: Then quoth the portress to the mistress of the house, "O my lady, arise and go to thy place that I in turn may do my devoir."  Sir Richard Burton, The Book of the Thousand Nights and a Night, vol. 1"
"1885: Then quoth the portress to the mistress of the house, "O my lady, arise and go to thy place that I in turn may do my devoir."  Sir Richard Burton, The Book of the Thousand Nights and a Night, vol. 1"