Absolute bums

They've always been called bums. Because that's literally what they were.
In the slums and poor neighbourhoods honest work was hard and low paid so some bums looked to the ring to avoid a regular job. So those who treated boxing as a real profession and worked hard and tried to win were given due respect. But those bums who just showed up for a weekend pay cheque, never trained for fights and flopped in the ring were bums. They were same guys who didn't work regular jobs. Just loafers who liked to fight - until fights got a bit tough when they'd quit.
Fighting for money isn't automatically honest work by any divine right. Thugs and bums fight anyway. It's not some middle class snobbery to point out the obvious.
And one of the reasons no one calls tennis players bums is because bums from the street corner or on a bar stool don't often fancy themselves as tennis players but they DO fancy themselves as fighters.
Sorry to break it to you but professional boxers has always had very low standards at the bottom. Unfortunately promoters are too keen to put bums in the ring.
In tennis you just don't get many fat lazy guys who haven't practiced in years getting invited to tournaments. It doesn't happen. And even the worst players have to run around for at least a couple of sets. When bums start turning up regularly on the tennis court, fat and breathless and quitting after a few points, I'm sure they would be called bums. But that won't happen.
Boxing has been called "the red-light district of sport" with good reason. There's a distinct underbelly of pro boxing that has no analogous cohort in something like tennis.

 

For those who never and would never call a fighter a "bum", what exactly would you call a fighter who is obviously in disgraceful shape and obviously looking to quit ?
Would you blanch at using the words "fat" and "lazy" and "quitter" ?
Are those words off limits to you too ?

 

Simmie Black was a memorable bum from the 1980s. He lost about 100 times at least and learned a few tricks along the way. He usually was stopped or took a dive after a few rounds. He fought under false names so no one knows how many times he fought. He even stole the identities of amateur boxers so he could fight with a clean record. This messed up the amateur status of those guys but Simmie needed his pay cheque and boxing commissions were clamping down on bums who were hoping to get knocked out every week, so Simmie didn't care.
He also used the identities of dead boxers. Simmie didn't care.
He was a real character and an absolute bum.
He was later convicted of sexual assault.

 

Rocky Balboa :
"It's just that you see I fight in clubs you know. I'm really a ham-and-egger and this guy's the best. It wouldn't be such a good fight."

 

I often use the term "set up" but that's relative to the quality of the fighter he's a set-up for.
A boxer who fought a lot of set ups is basically fighting men several levels below himself, so set-up isn't an exact absolute level itself.
One man's set-up might be a tough test for another man.

Bums are literally bums. They take dives, make no effort and don't train. They disgrace the sport. They don't mind losing, they hate training.

Stiffs are similar to bums. They don't fight back but some of them might just be too inept to get started. Stiffs might actually be fit and trained but they just aren't capable of fighting back.
Stiffs aren't as bad as bums but they might still take a dive when given half a chance.
Stiffs are suspect.