opak

1. wicked; evil; malicious
   U bajci se pojavljuje opaka maćeha. — A wicked stepmother appears in the fairy tale.
   Imao je opak osmeh na licu. — He had a wicked smile on his face.
2. (of illness, animal, opponent) dangerous; fierce; severe
   Preminuo je posle duge borbe sa opakom bolešću. — He died after a long battle with a serious illness.
   Pazi, komšija ima opakog psa. — Careful, the neighbour has a fierce dog.
3. slang badass; awesome
   Brate, ovo su opaka kola! — Man, that's one badass car!

Opaka bolest is often a euphemism for cancer.