The word "窃贼" is formed by combining the character "窃", which means "to steal" or "theft", with "贼", which means "thief" or "robber". "窃" semantically conveys the action of stealing, often implying secrecy, while "贼" refers to the person who engages in such criminal activity. Together, they create a compound noun where "窃" modifies "贼" to specify a thief who commits theft.