1- gaining significantmuscle mass 2- the act of drinking protein shakes, then hollaring "YEAH BEEF IT UP" , one may also do a series of push ups after drink has been chugged.
3- increasing anything in large amounts
1: The dogs had a great time at the park. They had a good time beefin' it out with each other.
2: Sorry bro, I know you're in the shower but is it cool if I take a squat and beef it out real quick?