sulud

1. (of an idea, action) insane; crazy; absurd
   To je suluda ideja, nemoj to raditi. — That's an insane idea, don't do it.
   Cene stanova u Beogradu su sulude. — Apartment prices in Belgrade are crazy.
2. (of a person) half-crazy; deranged
   Vozio je kao suludi čovek. — He drove like a madman.

Mostly used of ideas and actions; the adverb "suludo" is also common.