1911 Encyclopædia Britannica/Mentor

MENTOR, in Greek legend, the son of Alcimus and the faithful friend of Odysseus. During the absence of the latter, Mentor was entrusted with the care of his household and the guardianship of his son Telemachus. The word “mentor” is now used in the sense of a wise and trustworthy adviser, a meaning probably connected with the etymology of the name, from the root mon-, seen, in Lat. monere, to advise, monitor, adviser.

The New English Dictionary points out that the transferred use is due less to Homer’s Odyssey than to Fénelon’s Télémaque, in which Mentor is a somewhat prominent character.