naoštriti

1. to sharpen; to hone
   Naoštrio sam nož pre nego što sam isekao meso. — I sharpened the knife before cutting the meat.
   Moraš da naoštriš olovku, previše je tupa. — You need to sharpen the pencil, it's too blunt.
2. colloq. (naoštriti se) to gear up for a fight; to get worked up
   Naoštrio se na komšiju zbog parkinga. — He got all worked up against the neighbor over parking.
   Navijači su se naoštrili pred derbi. — The fans got fired up before the derby.

In the second sense it usually goes naoštriti se na koga — to turn against someone.