Answer: A Fox
When your elders yelled at you to stop all these games – or do mischief – they actually told you to stop yourself to behave like a smart fox.
The root of the word gimmick is bleak at best given the age of the phrase and potential competitors. However, among all competitors, the Irish term "sionnachuighim", which translates to "I play the fox", is the most convincing option as it has a similar sound and context. Not surprisingly, the Irish had an expression that linked the fox with mischief and deceit, as the fox appears as a world class mischief in dozens of cultures around the world.
In Finnish narratives, the fox is a fountain-significant fraudster; in both Japanese and Korean folklore, foxes are malicious and crafty spirits, and in Western and Eastern literature the fox repeatedly appears as a creature of great curiosity and mischief.