Haim’s Danielle Haim performs at a sold-out show Wednesday at the 9:30 Club. (Josh Sisk/For The Washington Post)

There are, certainly, a hundred possible ways to elegantly describe Haim’s performance at the 9:30 Club on Wednesday night. But probably none of them are quite as succinct or as spot-on as the four-word review one exhilarated fan uttered after the show ended:

“They’re so f------ cool.”

In 2013, the California-based family band — consisting of the three Haim sisters, Este (age 28), Danielle (25) and Alana (22), and their drummer, Dash Hutton — pulled off a well-documented, astonishingly quick transformation into young, hip America’s collectively agreed-upon favorite band, thanks to the pleasant pastiche — influenced by R&B, ’70s rock and ’80s pop — found on their debut album, “Days Are Gone.”

Read any profile of the band and you’ll quickly learn that the three sisters are geeky and self-deprecating. Their stage show, however, is an electrifying reminder that, refreshingly normal as they may be in their offstage lives, these women are anything but giggly girls next door when they make music. Their sold-out performance (there was a second sold-out show Thursday) was a formidable display of unapologetic awesomeness.

The first lights came up at the start of “Falling,” shining on lead vocalist and guitarist Danielle standing center stage in full rock-god power stance, feet apart and gaze startlingly intense. Thumping guitar and bass interludes between early numbers such as “If I Could Change Your Mind” and “Honey & I” found the three stomping, thrashing, slapping and pounding on their instruments while throwing their long hair around so violently and awesomely that Robert Plant could have taken some pointers.

Of course, their trademark goofiness was present, too. Alana and Este burst into a spontaneous rendition of Tenacious D’s naughty “Double Team” and after the bouncy “Don’t Save Me,” Alana playfully appraised the audience’s palpable enthusiasm: “That was a solid seven.”

“Give them a break,” Danielle cut in. “Isn’t it, like, 8 o’clock?”

What’s striking — and inspiring, even — about Haim’s show is its lack of performed prettiness. It’s still unusual, to say the least, for such a broadly popular female act to put so much emphasis on power and awe onstage and so little on adorability or sex appeal.

Which makes it feel cheap to also describe Haim’s sensational Wednesday night performance as “sexy.” But Haim’s onstage antics are unambiguously sexy — that is, exuding the same wild, sweaty, carried-away euphoria as sex itself, if not the pouty, glossy, performed sensuality of the average pop diva’s music video. During a particularly potent interlude in “Forever,” for example, Este un-self-consciously let her head roll backward and her jaw slacken; her eyes were squeezed shut, a few strands of her hair sticking messily to her mouth. No mistaking that.

But when the women of Haim turned their backs to the audience during songs such as “Running If You Call My Name,” it wasn’t to swivel their hips or look back over their shoulders alluringly. It was to stagger and stomp around erratically, so consumed by the music that it was as if they’d forgotten the concert­goers were even in the room. And it’s quietly thrilling to note that those concert­goers — who turned up in force and left afterward in varying states of elation — have fallen for them just the same.