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"The future is female," Hillary Clinton announced in her latest speech, immediately setting off a passionate debate between her supporters and critics. But by using the phrase, she also stepped into a virtual lesbian separatist history text — likely without having any clue about the story behind the slogan.

The former secretary of state and Democratic presidential candidate casually invoked the phrase in a video introducing this year's MAKERS Conference, an annual event hosted by AOL that brings together female leaders to create "a bold agenda," according to its website.

"Despite all the challenges we face, I remain convinced that yes, the future is female," Clinton said in the video, which was shown Monday. "Just look at the amazing energy we saw last month as women organized a march that galvanized millions of people all over our country and across the world."

Clinton's use of the phrase sparked a flurry of passionate tweets from conservatives who cried sexism, while it drew praise from women's rights advocates — some of whom mocked the strong conservative outcry.

"The Future Is Female" is not a message of equality, it implies dominance of one gender over another — Graymalkin (@Razgriz133) February 7, 2017

The Future Is Female is about equality, not about exclusion. Those precious Alt-Right Snowflakes have gotten their panties in a bunch again! — KillerTigger (@KillerTigger) February 7, 2017

The 1970s revisited

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If conservatives are upset now, wait until they find out that the phrase Clinton used grows out of 1970s lesbian separatist culture and is enjoying a vibrant second life today.

"The Future Is Female" became re-popularized in 2015 by the lesbian-owned lifestyle brand Otherwild, which started producing a line of T-shirts and other items featuring the phrase in stark lettering against a plain background.

Otherwild owner Rachel Berks told NBC News that she first came across "The Future Is Female" on the Instagram account @h_e_r_s_t_o_r_y, a lesbian culture archive.

The account's administrator had posted an archival photo of 1970s lesbian folk singer Alix Dobkin wearing a T-shirt with the phrase emblazoned across the chest. The photo was taken by Dobkin's then-girlfriend, Liza Cowan, who told ID magazine in December 2015 that she identified as a lesbian separatist and that the slogan "The Future Is Female" was a "call to arms" and an "invocation."

Kelly Rakowski, who oversees the popular Instagram account (and its more than 70,000 followers), told NBC News that she sees the phrase as a "feminist declaration."

"It's pretty wild to hear Hillary Clinton drop 'The Future Is Female,'" Rakowski said . "I never thought that what I unearthed in the depths of the internet would be such a broad, cultural sensation."

Berks reissued the design and the t-shirts took off; Otherwild expanded into pins, prints, bags, and even "Future is Female" coffee mugs. Knockoffs began to circulate as well, with controversy following model Cara Delevingne after she began to sell her own bootleg version of the t-shirt in violation of Berks' copyright in 2015.

Former Secretary of State Hillary Clinton at the Broadway musical "In Transit" last week in New York. Greg Allen / AP

Berks told NBC News it was "surreal" to hear the words come out of Clinton's mouth.

"It feels somewhat surreal to hear Hillary speaking these words, but is not surprising, given the current political climate and as female-identified bodies and rights remain under unrelenting attack," Berks said "'The Future Is Female' became a rallying cry throughout the course of the election, and most notably for me, after the election."

Berks also noted that Clinton mentioned the Women's March. Berks attended the march herself, waving a "future is female" flag, and said she was "floored" by the number of marchers holding handmade "future is female" posters.

Sales of the T-shirts raise money for Planned Parenthood, with 25 percent of proceeds going to the reproductive health organization.

Berks said she wasn't surprised by the conservative backlash to Clinton's use of the phrase.

"They want us without health care, without sliding-scale clinics like Planned Parenthood. They want to rescind the Violence Against Women Act," Berks said of the Trump-era Republican agenda.

Twitter conservatives also mocked "The Future Is Female" for talking about gender at all, with the editors of VDARE, a blog focusing on "patriotic immigration reform," pretending to be shocked by such old-fashioned notions.

The Future Is Female is trending.



But did... did you all just assume the future's gender? Wow, you disgusting bigots. BLOCKED. — Virginia Dare (@vdare) February 7, 2017

Rakowski acknowledged criticisms of the phrase, but she said its meaning went beyond flat readings of male and female.

"I think the word female can be less structured in this definition," Rakowski told NBC News. "Reading between the lines for me, it's really saying 'Smash the Patriarchy.'"