The message on the receipt rattled Sadie Karina Elledge, but it made her grandfather see red.

Instead of leaving a gratuity on Monday, a couple eating at the Harrisonburg, Virginia, restaurant where Sadie works scrawled: “We only tip citizens.”

The dig was aimed at Sadie, 18, who was born in America, but is of Honduran and Mexican descent. So, John Elledge took a photo of the grease-stained receipt left for his granddaughter and posted it on Facebook.

Beneath the photo he typed: “You are a complete and total piece of dung.”

Earlier on Facebook, the lawyer had written some other harsh words:

“I’d happily do the jail time if I could get just one solid punch in to the face of the son of a b---- who paid for his meal at the luncheonette where my granddaughter works and left the receipt for her with a note saying, ‘Sorry, we only tip citizens.’”

Elledge, who is white, said he’s particularly sensitive to slights directed at his multicultural family.

After “flunking out of college,” Elledge spent most of the 1980s in Honduras, working for the Episcopal church. He taught English to children in a bilingual school, started a youth program and met Iris, the Honduran woman who would become his wife.

She already had two children, so he adopted them and they returned to the United States. The family settled in Harrisonburg, a Virginia city ringed by three universities. The universities provide a lot of cultural diversity; so does a refugee resettlement office based in Harrisonburg.

It’s a good place to raise a blended, multicultural family, Elledge said.

“I’ve gotten six wonderful grandkids,” Elledge told The Post. “Sadie’s the third oldest. Her dad’s Honduran — my son — and her mother is Mexican. We’re a totally bicultural family. A pretty typical bicultural family.”

In three decades in a Southern city, they’ve had few discrimination problems, Elledge said. Once, in school, a kid told Sadie to “make me a burrito,” but it didn’t faze her.

Elledge says his grandkids are well-adjusted. Sadie plays soccer and posts selfies on her Facebook page. She graduated high school a few months ago and starts community college in a few weeks.

Elledge’s law firm is in the centre of town, and it allows him to dote on his grandchildren. One granddaughter works as a secretary in the firm, and Jess’ Lunch, the restaurant where Sadie works, is a few steps away.

After Elledge posted the receipt on social media, the story spread quickly among his friends, including a few lawyers and investigators who tried to identify the person who scrawled the signature at the bottom of the receipt.

But before they could figure it out, the people who ordered the gyro and the drinks returned to the restaurant, angered after seeing the post.

The man “was yelling and screaming about the four digits (of the credit card),” said Tom Marchese, the manager at Jess’ Lunch. “I said it’s not even your card. Are you really concerned about that or are you more concerned about what was put on social media? He said ‘well, both.’”

“I told him why is he even yelling at me, he should go to the person that did it,” Marchese said.

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About that time, John Elledge walked into the restaurant. He’d heard the people who wrote the nasty message to Sadie were back, and marched to the restaurant to meet them face to face.

“We didn’t talk much,” Elledge told The Post.” She was mad that I posted it ... the guy, he was being really belligerent.”

“ ... She was asking me why I posted it,” Elledge said. “I said obviously, it was an insult … against my granddaughter, darn right I’m going to post it. And no apologies.”