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")
In 2370, Zolan believed that Rugal's Bajoran parents referred to him every day as "Cardassian scum". (DS9: "Cardassians")
In 2373, Tom Paris called Harry Kim scum as part of an attempt to protect him from the other prisoners in the Akritirian maximum security detention facility. He claimed Kim had squealed on him in front of the Tribunal. (VOY: "The Chute")