Triangeljosti.
The Jostiband is a Dutch orchestra for people with a developmental disability, mainly people with down syndrome.
A [triangle](https://en.wikipedia.org/wiki/Triangle_(musical_instrument)) , or triangel in Dutch, is possibly the simplest instrument you can think of.
So calling someone a ‘triangeljosti’ is basically comparing them to someone who plays the simplest possible instrument in a band for developmentally disabled people.