What is the X in X-mas?
late 14c., "guardian, custodian," from Old French tutour "guardian, private teacher," from Latin tutorem (nominative tutor) "guardian, watcher," from tutus, variant past participle of tueri "watch over," of unknown origin. Specific sense of "senior boy appointed to help a junior in his studies" is recorded from 1680s.
1590s, from tutor (n.). Related: Tutored; tutoring.