The word 投诉 is formed by combining two characters, each contributing a key aspect of the overall concept. The first character, 投 (tóu), originally meaning "to throw", here carries the sense of "to submit" or "to present forward". The second character, 诉 (sù), means "to tell", "to relate", or "to complain". When fused, they create the compound idea of "submitting a complaint" or "lodging a grievance", where one actively presents or forwards a statement of dissatisfaction to an authority or relevant party.