NFL's DeAngelo Williams Stiffed Waitress, Says 'Service Was Horrible'

7:52 AM PT -- More bad news for the server ...

TMZ Sports spoke with Jimmy Marcos -- co-owner of Ledo Restaurant in Maryland -- who tells us DeAngelo's server will be TERMINATED over her handling of the tip situation.

Long story short ... we're told the restaurant policy is "the customer is always right" and tipping is a reflection of the service -- not mandatory.

"We were in the wrong," Marcos says.

He also has a message for Williams -- "I want to offer anything I can do. I'll be sure to apologize in person next time he visits and pick up his next meal."

Pittsburgh Steelers running back DeAngelo Williams is blasting restaurant workers who called him out on social media for not tipping ... saying point-blank -- THEY SUCKED AT THEIR JOBS.

It all started when the bartender at the restaurant went to Facebook and wrote:

"Just now at work I had Deangelo Williams come in and I waited on while tending bar. His check was $128.25. He left me $129 with no tip but .75 cents. So there you go Stealers fans, your running back is cheap as s**t!!! Smh."

Williams found out about the social media shaming and lashed back -- claiming the service was so terrible, a 75 cent tip was TOO GENEROUS.

"I tried to leave exact change but couldn't," Williams said ... "I waited on my food for over 1.5 hours."

(For the record, the waitress claims he was only there for 45 mins).

So, what was allegedly so bad?

For starters, "She never asked for refills," Williams says.

But what really pissed him off -- "I ordered chicken and shrimp in my pasta they put it on my salad."

When he pointed out the mistake, "They took it back and removed it from salad and put It in my pasta leaving a shrimp tail in the salad."

Williams says "terrible service" means terrible tip.

"I use to wait tables and use to be a bag boy so I know what I'm doing here."