I only had time for a short one this week, sorry.
I started reading The Odyssey a few days ago, and who do I come across in the first few pages? Mentor, Odysseus' trusted adviser, who is in charge of his household while he's off adventuring.
This has to be where the word "mentor" comes from.
One quick etymology online search later:
Mentor, "wise adviser," from The Odyssey. The word comes from Greek mentos, meaning "spirit, intent, purpose, passion," from root men- "to think."