oslabiti

1. to weaken; to grow weak
   Posle bolesti je mnogo oslabio. — He grew very weak after his illness.
   Vetar je do jutra oslabio. — The wind had weakened by morning.
2. colloq. to lose weight
   Oslabila si, baš lepo izgledaš! — You've lost weight, you look great!
   Oslabio je deset kila za dva meseca. — He lost ten kilos in two months.
3. to weaken
   Kriza je ozbiljno oslabila domaću privredu. — The crisis seriously weakened the domestic economy.
   Bolest mu je oslabila imunitet. — The illness weakened his immune system.

For loosening a belt, screw or knot, Serbian usually uses "olabaviti" or "popustiti", not "oslabiti".