Scum was a slang term for a reprehensible person.
The Clanton gang was described as scum by Morgan Earp. (TOS: "Spectre of the Gun")
Odo was described as scum while visiting Quark's in 2369. (DS9: "A Man Alone")
Aamin Marritza, described the prisoners at Gallitep labor camp, while impersonating Gul Darhe'el, as Bajoran scum or spineless scum. (DS9: "Duet")
Zolan believed that Rugal's Bajoran parents referred to him everyday as "Cardassian scum". (DS9: "Cardassians")