debilan

1. colloq. derog. moronic; idiotic; stupid
   Kakva debilna ideja, ko je to smislio? — What a moronic idea, who came up with that?
   Gledali smo neki debilan film do ponoći. — We watched some idiotic film until midnight.
2. dated (in medicine) feeble-minded; mentally deficient
   Stari udžbenici su takvu decu nazivali debilnom. — Old textbooks called such children feeble-minded.

A rude word; its medical sense is outdated, and in ordinary speech it is considered offensive.