He may be longing for a loftier official title, but Mayor Bill de Blasio has already been given a new nickname back at home — Bigfoot.

The 6-foot-5 mayor — who on Thursday became the 24th Democrat to announce a 2020 presidential bid — earned the moniker, sources say, for routinely swiping the policy plans of other politicians and touting them as his own, cribbing from colleagues including City Council Speaker Corey Johnson and fellow lefty Rep. Alexandria Ocasio-Cortez, among many others.

“People who say Bigfoot isn’t real haven’t seen de Blasio in action,” said one city official. “He’s always big-footed other elected officials to steal their ideas.”

A former de Blasio staffer told The Post, “I know he’s been called ‘Big Bird,’ but when it comes to governing, he’s more of a ‘Bigfoot.’

“He stomps in and takes other people’s ideas,” the staffer said.

A recent example of Bigfoot Bill is the city’s plan to provide free phone calls for Rikers Island inmates.

Johnson introduced the legislation over a year ago, in April 2018, to give jailbirds 21 minutes of free phone calls every three hours.

But on May 1 — two days before the bill was supposed to take effect — the mayor sent out a press release saying he’d just implemented the bill.

“This reform continues a string of initiatives by the de Blasio administration intended to make our jails safer, more accessible to friends and family and more equitable,” he crowed.

Johnson declined to comment.

Meanwhile, one of the most glaring examples of de Blasio appropriation is his use of the “Green New Deal” title for the city climate change plan he launched in April. It’s the exact same language that firebrand freshman Congresswoman Alexandria Ocasio-Cortez (D-Queens) used when she announced her proposal in February.

Yet de Blasio doesn’t like to dwell on that fact.

“Are you in touch with Alexandria Ocasio-Cortez about the federal version of this since you borrowed the name?” a reporter asked the mayor Monday at his chaotic Trump Tower press conference.

“Yeah, I support the federal bill, Congress member Ocasio-Cortez and Senator Markey’s bill. I’ve been very supportive of it. She’s been supportive of the New York City Green New Deal. Anything else?” an annoyed de Blasio asked, looking to change the topic.

And de Blasio has taken other pilfered policies on the road to pad out his stump speeches as he laid the groundwork for a 2020 run for president.

In Iowa in February, he told the Des Moines Machinists Union that “mayors have to get things done. Not talk, but action.”

At the Las Vegas Culinary Workers Union in April, he boasted, “I know one of the places that change can be done, I know we’re doing it in my city.”

He listed universal pre-K, paid time off, fossil fuel divestment and closing the Rikers Island jail as his signature achievements.

While de Blasio can undoubtedly take credit for free pre-K for all, Public Advocate Jumaane Williams first proposed paid vacation time in 2014, when he was a Brooklyn councilman.

So Williams was peeved in January when de Blasio’s staff gave him an hour’s notice to attend the mayor’s press conference on paid leave, a source told The Post.

“That’s funny, I’ve been carrying this bill for five years and he never expressed any interest,” Williams grumbled at the time, the source said.

Williams would only say, “I’ve been glad to see my paid vacation legislation receive the administration’s support, and I hope to see it become law soon.”

The proposal hasn’t even been scheduled for a vote. Yet de Blasio told the Vegas union workers, “Right now, this year, we’re passing a law in New York guaranteeing two weeks’ paid vacation for everyone who works.”

As for removing fossil fuel companies from the city’s $200 billion pension system, city Comptroller Scott Stringer, the custodian of the funds, is leading the effort to untangle fossil fuel investments from the system.

Indeed, de Blasio caused attendees at his State of the City speech in January to turn toward Stringer in disbelief when he took credit for the plan, a source said, adding, “Stringer is doing all the work.”

As for Rikers, de Blasio bragged in Vegas, “We have this big jail complex, that Rikers Island, we’re going to shut down and go to community-based facilities,” he said.

But Rikers won’t be closed until at least 2027 — eight years after the mayor leaves office. And it was former City Council Speaker Melissa Mark-Viverito who convened a commission to study the issue in 2017.

“The mayor is getting out in front of the band and pretending to lead,” said one frustrated councilman.

“A national audience would never know anything behind whatever rhetoric the mayor offered,” the fellow Democrat said.

De Blasio also used the council’s recent building emissions bills in an April 23 fundraising email from his national political PAC.

“Interesting,” said Queens Councilman Costa Constantinides, who sponsored the legislation. “I’m really glad the mayor sees the value in the work we’ve done,” he said sarcastically. He did, however, pointedly thank Speaker Johnson for his leadership.

And nearly three weeks after their April 18 passage, the mayor still hasn’t signed the bills into law.

“It’s ironic that fighting climate change is a major focus of the Democratic Party right now but he won’t schedule the time to sign what are arguably the most important bills in this fight right now at the city level,” a council source said.

The list of de Blasio’s big-footing stretches back years, including when in 2015, developers David and Jed Walentas of Two Trees Management pitched de Blasio on a streetcar route connecting the Brooklyn and Queens waterfront. Hizzoner’s reaction was “lukewarm at best,” sources said.

But he perked up after the Walentases gave $100,000 to de Blasio’s now-defunct Campaign For One New York and the mayor “realized he really didn’t have any big infrastructure projects” under his belt, a source said. By January 2016, he’d announced a $2.5 billion streetcar plan. The project has since stalled.

De Blasio also co-opted Manhattan Councilman Ben Kallos’ campaign finance reforms, Brooklyn Councilman Mark Treyger’s proposal to increase the number of translators at city poll sites, and Speaker Johnson’s safe injection sites for drug users, sources said.

“It’s hard to think of anything progressive that he’s had to push through the council,” another member said. “It’s all been the other way. It’s all been the council pushing him to adopt progressive priorities.”

Not denying that he co-opted the ideas of others, mayoral spokeswoman Freddi Goldstein told The Post, “Not one of these things would have happened without the mayor’s leadership. We’re proud to collaborate with other elected officials so that New Yorkers can reap the benefits.”