In contemporary speech on both sides of the Atlantic the word ‘mentor’ is typically pronounced with a full second vowel despite the fact that the meaning is agentive and, therefore, should be pronounced with a reduced second vowel (as in the agentive suffix -er). The spelling may have something to do with it, but the more persuasive reason resides in the word’s structure, viz. the lack of a morphological boundary between ment- and -or. Which is to say that speakers do not interpret the word as a true agentive despite its obvious meaning.

MICHAEL SHAPIRO