What is "beefing"

Usage examples of "beefing".

Rock did remember the day he had faced his father down and—after years of beefing up at the gym, at army boot camp, and on the first and only leave he had used to return home—with one stunningly quick left hook, had set him down on the pavement in front of the Pittsburgh ghetto tenement in which Rock had been brought up.

I had notified him that I intended to take Thursday afternoon off for the opening of the baseball season at the Polo Grounds, and when Thursday came I didn't want any beefing about my letting things pile up.