From what I understand, the whole bear movement had it's roots in the height of the AIDS crisis, when guys were dying at alarming rates and those who weren't dying were seemingly being eaten alive by a virus that left them emaciated, skinny, and unhealthy looking. Men started to look at the more robust of us as signs that we were healthy, or more to the point uninfected.

I remember using phone sex line back in those days and I always referred to myself as stocky, husky, or chubby. It never occured to me to use, nor did I ever hear the term, bear. (We're talking about 1990/91 here.)

American Bear magazine has back issues all the way back to 1994 (those are sold out) but presumably the word has it's roots somewhere between 1990 and 1994, perhaps earlier, just because I didn't hear about it doesn't mean it didn't exist, but I was pretty active in the gay community back then.

The first chapter of Girth and Mirth appeared in 1976, according to this article http://www.glbtq.com/social-sciences/bear_movement.html but they do go on to say that Bear Magazine was the first entity to use the word "bear."

Cheers
Dzinerbear
Bear Universe