Someone who's beefy has a big, dense, muscular body. Your beefy, loud-voiced gym teacher might turn out to be a sweet and thoughtful person.
The adjective beefy is a bit derogatory — in other words, it's not exactly used as a compliment. If you describe a person as beefy, you imply that not only is he large, strong, and brawny, he might also be somewhat dense. You could also use the word to mean "tasting like beef," although that's much less common. The word dates from 1743, from the sense of "human muscle."