slabiti

1. (+ acc.) to weaken; to debilitate
   Stres slabi imunitet. — Stress weakens the immune system.
   Ta odluka slabi poziciju vlade. — That decision weakens the government's position.
2. (without an object) to grow weak; to weaken
   Dinar ovih dana slabi u odnosu na evro. — The dinar has been weakening against the euro these days.

The perfective is oslabiti; in the intransitive sense slabeti is also used.