My mother told me the story of how, when SHE was young (practically takes us back to B.C!) , she'd visited one of those "doorin' talkies"(touring talkies), which were makeshift bamboo or palm-thatch shacks, with the movie screened on a large cloth.
The movie would go on, and a comedy scene would appear on the screen. The audience, she said, watched in silence.
Then, on the screen would appear the words, "Ithu jokku" ("This is a joke"). Immediately, the audience would erupt into laughter.
I strongly suspect my mother of having made the whole thing up, just to get a laugh out of us, but the word "jokku" and the phrase, "ithu jokku!" have been gleefully used in our family ever since.