kretenski

1. colloq. derog. idiotic; moronic
   To je bilo potpuno kretensko pitanje. — That was a completely idiotic question.
   Ko je smislio ovo kretensko pravilo? — Who came up with this moronic rule?

2. colloq. derog. idiotically; like an idiot
   Ponašao se kretenski celo veče. — He acted like an idiot all evening.

A rude word; glup is milder but can still be insulting.