This context is from the movie "on The Waterfront" The people speaking talk about a boxing mach in which one was a contestant and the other a spectator in the audience.
person 1. Man, he really dumped you.
person 2. He dumped me?
person 2. What would you say if I told you I held that bum up for half a round?
person 1. I could see he was hurt.
person 2. What did you think I was doing with them combinations, petting him?
TFD definitions for the phrasal verb "hold up"
2. verb To physically keep someone or something upright. In this usage, a noun or pronoun can be used between "hold" and "up."
He was so sick that I practically had to hold him up in the pharmacy line.
3. verb To delay or slow someone or something down. In this usage, a noun or pronoun can be used between "hold" and "up."
But these definitions don't make much sense because further converstation indicates that he lost that fight on purpouse because "he was doing a favour to couple of palls of his" also he brags about having his opponent "figured out" and that if it wasn't for the favour he would've won.