a. A hairy (esp. chest)ed gay man, usually of middle years or more, who is well-muscled or well defined ("cut")usually from body-building or progressive-resistance gym work, with visible attributes such as forearm "guns" or "six-pack abs."
b. More generally, any hairy-chested mature (usually but not definitively) gay male who is at least somewhat physically fit, especially one who presents an imposing or dominant presence. Facial hair and a blue-collar look such as the cliche plaid lumberjack shirt add to the image.
But to me, the quintessential Muscle Bear is Tim Kelly in the HOM gay-porn vids. Woof!"
(Definition b) -- "Mary's straight-as-an-arrow husband Lochinvar is six foot one, hairy, a little chunky but still in good shape from outdoor work. He's forty-three and wears a goatee. Is it safe to call him a muscle bear?" -- "Well, you'd better check it out with Mary to see if he would get upset at any gay inference. But if Mr. L. grows a beard and starts hanging out in taverns every evening, perhaps Mary should start worrying. And why are YOU so concerned, might I ask?"