Izlupati nekoga or nalupati nekoga is "to beat someone". Lupati is "to talk nonsense", nalupetati se (it's allways reflexive) is "to say a lot of bullshit" :)
Primeri: Šta lupaš? - What a hell are you saying? or That's nonsense.
Ala si se nalupetao! - You said a whole bunch of nonsense.