Louis C.K. performed an unannounced show for about 50 fans at the Big Hunt in D.C. on Jan. 12. (Sean Joyce/Underground Comedy)

Of the 50 or so people who packed into The Big Hunt’s satanic-themed basement bar on Thursday night for a comedy show, most had no idea that they were about to see Louis C.K. They likely didn’t even know they were going to a comedy show until a little earlier that day.

C.K. dropped in at the Dupont Circle bar for a surprise 15-minute set at a late show co-headlined by New York comedians Joe List and Mark Normand. Tickets, which cost $5, had stealthily gone live at midnight Wednesday. Midday Thursday, D.C. stand-up promoter Underground Comedy announced the show — teasing that there might be special guests — in an email and on social media, and tickets sold out by 4 p.m.

After short sets from Underground Comedy founder Sean Joyce, Virginia’s Chris Alan, List and Normand, C.K. emerged to a stunned — and raucous — crowd. “How loud it was when he went onstage — that’s twice as loud as I’ve ever heard that room,” says Joyce, who books eight shows a week at The Big Hunt.

C.K. opened his set by asking the audience if they’d ever thought about how many men their moms have had sex with. He later used hockey helmets in a metaphor about homophobia, cut down a heckler and shared a bunch of random, compact jokes (including one inspired by one of his daughters’ observation that whistles are dirty). He often consulted his notes and was dressed casually in a black sweater and jeans, a contrast to the suit he’s been wearing for most of his advertised stand-up shows.

C.K. is in the middle of a four-night stand at DAR Constitution Hall, where he’ll film his next stand-up special Saturday; List and Normand are among his opening acts. (All three performed at the venue before Thursday’s Big Hunt show.)

C.K. has spent a lot of time in D.C. over the past year as he’s toured the world polishing his latest set of material. In April, he did a trio of surprise shows at the DC Improv and Lincoln Theatre that were announced hours before they happened. He returned for more traditional shows at Constitution Hall in July and November.

His intimate set at The Big Hunt was loose, raw and completely different from the material he performed at his July D.C. date. Either C.K. was still working out parts of his special, or he was just letting out a few jokes that could form the basis for his next batch of material.

“Having Louis sit at the table in the back and scribbling notes down and talking to Joe about the jokes and then going onstage and trying them out — that’s what every single comic does,” Joyce says. “They sit at that table and they scribble in their notebook and they go onstage, Watching Louis do the exact same thing is, I think, very important to all the comics.”

C.K., who was born in Washington, is the latest comedian to make a surprise appearance at a D.C. bar in the past couple of weeks. Gaithersburg, Md., native Judah Friedlander dropped in at a couple of Joyce’s shows the week between Christmas and New Year’s Eve, and Patton Oswalt, who grew up in Northern Virginia, performed at an Underground Comedy show at the Wonderland Ballroom in Columbia Heights on Jan. 1.

Joyce, who wasn’t sure if C.K. would show until five minutes before the show started, says he doesn’t expect C.K. to drop in at any of the shows he’s promoting this weekend.