A “disgruntled client” stabbed a 17-year-old boy volunteering at a Brooklyn soup kitchen — possibly in a fight over a pitcher of apple juice, cops and sources said Thursday.

The teenager, who worked at Masbia of Flatbush as part of a summer youth program, was knifed in the stomach just outside the charity facility around 3:30 p.m. Wednesday, according to police and sources.

The broad-daylight bloodshed followed an argument that unfolded a short time earlier inside the soup kitchen, which has been open on Coney Island Avenue near Avenue J in Midwood for about 20 years with little trouble.

A source involved with the soup kitchen said the unhinged attacker may have flipped out because of an issue with beverages served during the facility’s dinner service, which begins around 3 p.m., and included a meal of rice, beans and tuna that day.

“The pitcher of apple juice was not filled to the top on that table — like, that sort of thing,” said the source, who arrived at the facility after the skirmish. “When we open, it’s the busiest time. That’s the peak.”

The aggressor became “mouthy to one volunteer” before another staffer warned him he’d have to leave if his behavior continued, the source said. 

“Instead of that de-escalating, it escalates,” the source said. ”And then it spilled over to outside.”

A volunteer who declined to give his name said he got to the soup kitchen for his shift and heard “people quarrelling” and “getting more boisterous” until about four workers, including the victim, got the unruly person outside.

“They were exchanging words, fighting … I was holding him back,” the witness said. “Then the knife came out.”

The teen, who was knifed in the lower right side of his stomach, “was saying ‘I’m stabbed. I’m stabbed,'” the fellow volunteer recalled, describing the injury as a minor wound. The boy was nearing the end of his six weeks working at the facility.

He was taken to Maimonides Medical Center, where he was listed in stable condition.

The attacker, who was wielding what the volunteer described as a “small little knife” akin to “a multi-purpose tool,” is someone who occasionally eats at the soup kitchen, the worker said.

“It was unnecessary, what happened was unnecessary,” he added, decrying the violence and lack of police presence in the area.

The suspect was still on the loose Thursday.

“A soup kitchen was a safe place and will stay a safe place,” Alexander Rapaport, executive director of Masbia, told The Post. “And even in safe place, vigilance is important, and de-escalation is important.”

“The takeaway of this incident is emphasizing more de-escalation with both staff and volunteers and doing it right.”