The use of the word not with words beginning with un - obscures meaning with excess negatives. Even the use of the word not by itself tells only what a person, place, thing or idea is not, rather than describes the noun accurately in direct terms. This famous quote tells you only that he has no criminal record. Specialized professions and fields of study all use jargon to make subtle or technical distinctions between similar concepts. For example, the word cancer has only one meaning for most people outside of the health and medical fields.
Malignant melanoma communicates far more information to a doctor than just the word cancer .
If I've made myself too clear, you must have misunderstood me. But writers and speakers can intentionally obscure meaning with jargon in order to intimidate or to baffle the listener. Many people feel reluctant to admit ignorance. The use of highly specialized words and expressions can cause the reader to feel less intelligent or less knowledgable than the writer or than other people in general. The unnecesary use of jargon often goes unchallenged.