debljati

1. (debljati se) to gain weight; to put on weight
   Od kada je prestao da trenira, stalno se deblja. — Since he stopped training, he keeps putting on weight.
   Zimi se svi malo debljamo. — We all gain a little weight in winter.
2. (of food, clothes) to be fattening; to make someone look fatter
   Ova haljina te uopšte ne deblja. — This dress doesn't make you look fat at all.
   Kažu da beli hleb najviše deblja. — They say white bread is the most fattening.

In the sense "to gain weight" it is usually used with se (debljati se), but it also occurs without it: debljati (se).