I guess the English language has changed since 1946 when this thesaurus was published, or I’m really stupid. I never heard of this meaning for the word mean (as a verb: average; as a noun: medium, happy medium, average, balance, normal). But then I looked in The Synonym Finder, and there’s happy medium in there too!
And here I thought if you weren’t nice, you were mean. Or what I mean to say is if you’re not nice, you’re kind of a bully. Or they don’t have any means to go by in order to change.
food for thought: I don’t know why people are mean. I’m not sure what they have to gain by being so. But did you know that mean is a happy medium?