You havin a laugh is a long-standing British colloquial way of asking if someone is joking, roughly meaning you cannot be serious. Online, the short audio version spread as a reaction clip on soundboards and in edited videos, where creators reuse the same disbelieving delivery for many different situations. It sits alongside other short question reactions like you what, because both let an editor answer a scene without recording new dialogue. Its popularity comes from how specific the accent and attitude feel while the meaning stays instantly clear to a wide audience.