One hundred fifty-six years ago Wednesday, a theater full of people laughed the most consequential laugh in American history. A murderer knew exactly when they would do it and used the noise to cover his assassination of the president of the United States.

Here is the line that prompted their hilarity: “I know enough to turn you inside out, old gal, you sockdologizing old man-trap.”

Upon delivery, the house exploded. But today, it’s baffling — unfunny, seemingly unworthy of its place in history alongside Abraham Lincoln’s death. An indignity.

We tend to think of people from long ago as credulous, unsophisticated fuddy-duddies. Like our grandparents, only lamer. So we read the fatal joke and conclude the audience was just . . . wrong. In her travelogue “Assassination Vacation,” Sarah Vowell finds wry humor all over history yet dismisses this line as indecipherable. She pities the park rangers who have to quote it on tours of Ford’s Theatre.

In fact, we’re the ones who have it wrong. The last words Lincoln heard were funny. And smart. Humor on stage is about context, which the sentence lacks when isolated in writing. Live theater is also about delivery and the relationship between actors and audience, which develop over the performance. Literally, you had to be there.

Maybe it’s time to give that 1865 audience back its dignity.

Our American Cousin,” a play written by British polymath Tom Taylor, is a fish-out-of-water comedy like “The Beverly Hillbillies” or “The Fresh Prince of Bel-Air.” A Vermont hick named Asa Trenchard — a blunt, boorish, American sort — appears among posh English society as the heir to a family fortune. He refuses to take a bath, says “skedaddle” and calls milk “cow juice.” But he is savvier than he seems.

The punchline moment arrives as something of a comeuppance for Mrs. Mountchessington, who is exactly as you would picture a Mrs. Mountchessington. She hides her contempt for the American, schmoozes and flatters him. She wants him to marry her daughter and promises they don’t care about his money, just true love.

He announces that he is delighted to court Mrs. Mountchessington’s daughter for love alone, since he has renounced the inheritance and will never be rich. Mrs. Mountchessington’s disappointment is palpable, as is her greed. She beats a hasty retreat.

After she leaves — as John Wilkes Booth waits — actor Harry Hawk, alone on stage, delivers his line. Sockdologizing. Man-trap. Fifty feet away, stage left, house right, four yards high, Booth oozes into the president’s box, unheard.

Onstage, any remaining doubts about Asa Trenchard are obliterated. One line confirms what the audience suspected: The clodhopper is in fact a shrewd judge of character. Add to that a dose of naughty sauce (“man-trap”) — always good for a snicker. And “sockdologizing”? Sounds funny, for one, and it was downright elegant. The playwright seems to have invented a neologism derived from American slang of the time: A “sockdologer” meant a truth delivered as a defining moment in a situation, an intellectual coup de grace. Altogether, those 14 words are a checkmate, voiced in Trenchard’s vulgar patois. Voilà: He is crude, but a merry cynic. A quick, deep joke. Bang!

So to speak.

If you’re not laughing, it’s likely because you are not in the moment. You were not watching it happen. How to provoke a guffaw loud enough to cover a gunshot? A showstopping laugh line takes careful setup, possible only when an audience knows the characters and trusts the show. By Act 3, Scene 2, playgoers are invested. They’ve let down their guard. From individuals, an audience is forged.

There was also the context outside Ford’s Theatre: 156 years ago, the Civil War had been over for just five days; the previous week, Lincoln had walked through Richmond — the fallen capital of the Confederacy — only lightly guarded. Safety and normalcy were returning, and a little frivolity, a hit comedy from overseas, was a relief.

It also happens that performers’ descriptions of getting a huge laugh — slayed ’em, I killed, punchline — echo violence. And experiencing such mirth sounds like injury: split my sides, in stitches, busting a gut.

If your whole job involves knowing how to ease an audience into a vulnerable, relaxed state, you know where humor can punch them with maximum precision — and precisely when nobody in the president’s box would be alert to a threat. Booth, an actor, was in the business of knowing audiences. It made him the most dangerous man in Washington that night.

“Other than that, Mrs. Lincoln, how was the play?” is a joke about missing the point, but Harry Hawk wrote to his parents that it really had been going well that night, that Mrs. Lincoln had been laughing all the way through and especially liked that line.

It’s too bad, in a way, because it’s hard to imagine anyone laughing at it ever again.

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