The manga moment in question occurs at the end of the Swordsmith Village Arc when all is said and done in terms of the big battle, and Nezuko has acquired not only the ability to stand under the sunlight but has also regained her ability to verbalize words. More or less.
In this one-panel scene, which fans have discussed on Reddit, Inosuke is attempting to make Nezuko say his name correctly, which she apparently struggles to do, as she erroneously exchanges the "N" of Inosuke for an "M." As u/Nicckgoat and others comment, this is ironic judging by the fact that Inosuke can only get someone's name right on every 7th attempt. To the boar-headed demon slayer, Tanjiro Kamado has been, among many other names, "Kamaboko Gompachiro," and Zenitsu has been "Monitsu." That is not to say that Inosuke does it to spite them. Amusingly enough, he's truly incapable of getting names right most of the time.
"That's the pot calling the kettle black," commented u/GirafeAnyway.
"I'm so excited. It's going to be the best," wrote u/StolenPens, "Zenitsu crying. Inosuke fighting. Nezuko being the best girl. I'm not 100% sure that it's not Nezuko punking him either. It's hilarious."
Indeed, the whole reunion between a now-talking Nezuko and her brother's two buddies, Inosuke and Zenitsu, the latter of which is infatuated with her and nearly has a heart attack, is a hilarious event in Koyoharu Gotouge's manga and likely to be just as or more amusing if and when voiced and brought to life on the screen. It won't be too long until fans confirm how much of this sequence will be included in Ufotable's final product.
