A term made popular by the Jerky Boys in the 90's. It's a put down that describes either a fat guy with large he-tits or a body-builder with huge pectorals.
You think you're a tough guy, huh beefy tits, fruity-ass...
When a female breast consists of more muscle than fat. Pretty nasty if you ask me...
"WTF?! That powerdyke has beefy tits! She must be one of them nasty body builders!"