It is term that came about back when Steely Dan
released a tune that had a chord that was called
the...moo major.
I imagine somebody was told about the chord then
forgot what it was called and said something like...I use to
know what it was called and it sounded beefy.
"Beefy" is a powerful sound dialed in with a good amp that
has a pickup that is capable of delivering a full range of clear tones.
Then with a good eq one can dial in most any sound they're capable of dialing in.
Contrary to what some believe it is a sound that can be made with single coil pups too.
I've heard some good guitars sound like sh!t because of bad amps.
I've also heard some good guitars with good amps sound bad because the player didn't
know how to adjust the pups or the wrong caps were killing the pups.
"Beefy" is more than just the pups IMO. YMMV