Prolix (Adjective) means (of a speech, writer, etc.) using too many words and so boring to listen to or read : a prolix speaker. Her style is tediously prolix. Prolixity is the Noun form of the word prolix. So, the word prolixity and brevity are clearly antonymous. Brevity means shortness or briefness; conciseness. For example, He is very famous for the brevity of his speeches.