kupaći kostim

1. swimsuit; bathing suit
   Ne zaboravi kupaći kostim, idemo na bazen. — Don't forget your swimsuit, we're going to the pool.
   Kupila sam novi kupaći kostim za more. — I bought a new swimsuit for the seaside.

It usually means a women's swimsuit; men's trunks are usually "kupaće gaće"; in speech it is often shortened to "kupaći".