šljam

1. derog. (of people) scum; rabble
   U tom kafiću se okuplja sav šljam iz kraja. — All the scum from the neighbourhood gathers in that café.
   Ne želim da se družiš s tim šljamom. — I don't want you hanging out with that rabble.
2. (at the bottom, in water) sludge; sediment
   Na dnu bazena nakupio se šljam. — Sludge has built up at the bottom of the pool.

In the sense referring to people it is a collective noun: used in the singular to refer to a group of people.