Don't get involved in any shenanigans!
The word "Fisematenten," sometimes also spelled "Fisimatenten," is used to warn someone not to get into trouble. If someone tells you to not do any "Fisematenten," they are warning you to stay out of trouble and not get involved in any nonsense.
"Fisematenten" itself doesn't really mean anything in German. In fact, many misspell the noun as "Fiesematenten" because it is mistakenly associated with the German adjective "fies," which translates to "mean" or "nasty." But the word is actually based on a mistranslation of a French expression from the early 19th century.
The most common etymological explanation is that the expression "Fiesematenten" is derived from the times of the Napoleonic Wars, when French soldiers invited young German women to visit them in their tents.
Most Germans didn't understand French at the time, but German parents had a feeling that their daughters' sudden pregnancies had something to do with the previous offerings of "visitez ma tente" (visit my tent). So they warned their daughters not to do any "Fisematenten" which to them sounded like "visitez ma tente."
Nowadays the word is also used as a synonym for "nonsense" or "shenanigans." Parents will for instance tell their children: "Enough with the "Fisematenten," it's time to go to bed."
So, no more shenanigans and enough with the "Fisematenten." It's time for you to practice using a new German word!